DEVELOPMENT OF THE LOW COST ADSORBENTS FROM ...

DEVELOPMENT OF THE LOW COST ADSORBENTS FROM

POLYSACCHARIDES-BASED BIOMASS FOR THE

RECOVERY OF GOLD

March 2014

Department of Science and Advanced Technology

Graduate School of Science and Engineering

Saga University, Japan

BIMALA PANGENI

Development of the Low Cost Adsorbents from

Polysaccharides-based Biomass for the Recovery of Gold

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF ENGINEERING

(D.Eng.)

by

BIMALA PANGENI

Department of Science and Advanced Technology

Graduate School of Science and Engineering

Saga University, Japan

March 2014

The Dissertation Submitted herewith is approved by the following Members of the Examination

Committee:

------------------------------------------------------------------------------------------------

Professor Keisuke OHTO

Chair of Advisory Committee

Department of Science and Advanced Technology, Saga University

---------------------------------------------------------------------------------------

Professor Hideyuki NOGUCHI

Department of Science and Advanced Technology,

Saga University

---------------------------------------------------------------------------------------

Professor Toshiyuki TAKAMUKU


Saga University

---------------------------------------------------------------------------------------

Associate Professor Shintaro MORISADA


Saga University

OVERVIEW OF THE THESIS

The wide application of gold to make traditional jewelries as well as in different

electronic devices such as computers, mobile phones, and other high tech electronic appliances

increases its demand every year thus the gold ore are continuously mined to fulfill its demand

which will invites the trouble of gold ore exhaustion in near future. The customer are expecting

more and more advanced or improve technology thus more sophisticated and technologically

advanced electronic devices were invented that replaces the old electronic devices thus large

amount of electronic waste is generated every year. In addition, the conventional cyanide

treatment for the extraction of gold is toxic that produces large volume of toxic cyanide solution

which is not environmentally benign. The invention of new recovery method of gold from

primary (gold ore) and secondary (wasted electronic devices) sources with convenient, low cost,

and environmentally benign technology is required. From the literature it was found that

cellulose is most abundant material in the world which is found mostly in plants. In the present

work, new recovery method was invented for the recovery of gold from trace concentration by

using chemically modified cellulose derived from agricultural plants, and compared with

commercially available cellulose together with some other polysaccharides contains similar

functionality such as dextran, alginic acid, and pectic acid. It was found that natural and

commercial cellulose containing hydroxyl functional group showed very high affinity and

selectively adsorbed gold from trace concentration even in the presence of other coexisting base

metals and precious metals. The traditional and industrial values of gold, conventional way of

recovering gold their disadvantages are systematically described together with the advantages of

novel, economic and environmentally benign technology investigated in this research work in

Chapter 1.

The novel material for Au(III) recovery was developed from commercial cellulose by

cross-linking with concentrated sulphuric acid. The cellulose is most abundant material in the

world. The aim of this research work is effective utilization of cellulose and its derivatives for

the recovery of gold from aqueous medium. The research work was conducted with aiming the

development of new adsorbent which may be successful to adsorb and subsequently reduced the

Au(III) to its elemental form without adding any types of reducing agents. For that,

commercially available cellulose powder was first cross-linked with concentrated sulphuric acid

in order to create the new coordinating sites for Au(III) adsorption. During the treatment,

crystalline form of commercial cellulose which have very low affinity with Au(III) ion was

converted into amorphous form with very high affinity for Au(III) ion. The trivalent gold was

selectively adsorbed onto the cross-linked cellulose gel from the mixture of other precious metals

and base metal. It was visually observed that yellow shining metallic particle was formed and

floating at the surface of the reactor at the short contact where as it was found to be aggregate

then forming heavy particle after long time contact which is settled down on the bottom later. It

was further confirmed from the observation of crystalline peaks of elemental gold [reduced gold

Au(0)] in XRD spectrum of Au- loaded cross-linked cellulose gel. So, based on the results, we

further expect to attempt whether such type of quantitative and selectiveness of gold adsorption

is common to all kinds of polysaccharide or not, we employed commercially available

polysaccharide such as dextran, alginic acid and pectic acid also hydroxyl as a major functional

group by cross-linking with concentrated sulphuric acid similar to the case of cellulose gel. In all

the cases there is high selectivity for Au(III) ion from the mixed solution thus the polysaccharide

based adsorbent investigated in this study can be expected to be promising material to be

employed in large scale industrial processing for gold recovery.

Although commercial cellulose after cross-linking yield very effective adsorbent for

Au(III), we have tried to extent our technology for the other low cost cellulose rich biomass thus

adsorbent was prepared from cotton by the similar method of cross-linking as in Chapter 2 and

investigate its adsorption behavior in Chapter 3. In Chapter 4 more abundantly distributed

waste paper was utilized for the recovery of gold together with the mixture solution of other

precious and base metals from acidic chloride media. Both the materials after cross-linking with

concentrated sulphuric acid selectively adsorbed gold with extremely high adsorption capacity.

Comparing with cotton, waste paper was much cheaper and abundant. So we are intended to

elaborate our technology to more abundant cellulose rich material like paper because Japan is the

third largest country for paper production in the world thus raw paper for adsorbent preparation

was easily available in low cost in Japan in comparison to cotton biomass.

Although we have succeeded to recover the trace concentration of Au(III) from hydrochloric

acid media by using cross-linked polysaccharide gels, our final target is to apply our technology

in actual industrial processing. The sulphite and cyanide salts of Au(I) are most common

chemicals used in gold plating industries where large amount of waste solution containing trace

amount of Au(I) is produced. Moreover, for the extraction of gold from its ore or some other

secondary sources like waste electronic appliances, cyanide leaching is most effective and

common. The gold plating waste solution and cyanide leached liquor of gold ore contains gold in

the forms of Au(CN)2- or Au(SO3)2

-, thus recovery of gold from such a solution is required. The

gold cyanide is very toxic compared to gold sulphite solution. It was found from the literature

that Au(CN)2- or Au(SO3)2

- possesses chemical similarity thus forms similar types of complex in

aqueous solution. From the results of Chapters 2, 3 and 4, it was found that the cross-linked

polysaccharide adsorbents were very much effective for Au(III) recovery in hydrochloric acid

media but the industrial gold plating waste solution contains anionic complex of Au(I) sulphite

or Au(I) cyanide. So that we further tried to recover mono-valent gold using cross-linked

cellulose gel from Au(I) sulphite solution in sodium hypochlorite media in Chapter 5. Hence,

the new way of recovering trace concentration of Au(I) from less toxic Au(SO3)2- solution has

been investigated and expected also to be successful to recover Au(I) also from cyanide medium

too. By using cross-linked pure cellulose (CLPC) gel, the Au(I) was successfully recovered in

oxidizing environment in sodium hypochlorite medium. It was believed that Au(I) in Au(SO3)2-

solution was oxidized into Au(III) with the aid of NaClO which is effectively adsorbed onto the

cellulose gel according to the method described in earlier chapters. For the application of

investigated polysaccharide based adsorbent in actual practice, the real leached liquor of gold

and silver was prepared from both primary source (Mongolian gold ore soil sample) and

secondary source (scraps of plasma TV monitor) and successfully recovered the gold and silver

by leaching with acidic-thiourea followed by its adsorptive recovery using cross-linked cellulose

gel is presented in Chapter 6 and Chapter 7, respectively. Finally, the overall concluding

remarks and an outlook are suggested in Chapter 8. At last part of this thesis, a list of

publications related to author’s works, list of presentation and contributions to scientific forum

are summarized in appendices.

i

Table of Contents

Page

Overview of Thesis

List of Figures ……………………………………………………………………………I

List of Tables …………………………………………………………………………….IX

List of Schemes………………………………………………………….……………….X

Acknowledgements………………………………………………………………………XI

Abstract…………………………………………………………………………………..XIII

CHAPTER 1

Introduction and Objectives of the Present Research Work 1-31

1.1 RESEARCH BACKGROUND ................................................................................................................ 1

1.1.1 Precious Metals ...........................................................................................................1

1.1.2 Platinum Group Metals (PGM) ...............................................................................3

1.1.3 Gold and Silver ...........................................................................................................3

1.1.3.1 Silver ..................................................................................................................... 4

1.1.3.2 Gold ...................................................................................................................... 4

1.2 WASTE ELECTRONIC DEVICES OR E-WASTE………………………………………………..6

ii

1.3 CHLORIDE CHEMISTRY OF PRECIOUS METALS (GOLD, PLATINUM AND

PALLADIUM) .................................................................................................................................. 8

1.4 PRECIOUS METALS RECOVERY TECHNIQUES ................................................................. 10

1.4.1 Recovery of Precious Metals by Hydrometallurgical Processes ............................11

1.4.1.1 Leaching of gold .............................................................................................. 11

Cyanide leaching ........................................................................................................... 12

Thiourea leaching .......................................................................................................... 12

1.4.1.2 Recovery of gold from leachate...................................................................... 13

1.5 POLYSACCHARIDES ................................................................................................................. 16

1.5.1 Chemical Cross-linking .................................................................................................... 19

1.6 OBJECTIVES OF THE PRESENT RESEARCH WORK ......................................................... 19

1.7 OUTLINE OF THE THESIS ....................................................................................................... 22

REFERENCES CITED ............................................................................................................................ 24

PART-I

Chapter 2

Chemical Modification of Some Commercially Available Polysaccharide for

Selective Recovery of Gold from Acidic Chloride Media 32-60

2.1 INTRODUCTION ............................................................................................................................... 33

2.2 EXPERIMENTAL PROCEDURE .................................................................................................... 34

2.2.1 Materials .................................................................................................................................. 34

iii

2.2.2 Preparation of Cross-linked Polysaccharide Adsorbents ............................................. 34

2.2.3 Measurement and Analysis .................................................................................................. 35

2.2.4 Batch-wise Mode of Adsorption Test ............................................................................... 36

2.2.5 Measurement of the Degree of Crystallinity of Cellulose ............................................. 37

2.3 RESULTS AND DISCUSSION ......................................................................................................... 37

2.3.1 Effect of Hydrochloric Acid Concentration for the Adsorption of Metal Ions ...... 37

2.3.2 Surface Analysis of the Adsorbents ................................................................................... 38

2.3.3 Adsorption Isotherm Studies ............................................................................................. 40

2.3.4 Effect of Temperature for the Adsorption of Au(III) onto CLPC Adsorbent ....... 43

2.3.5 Adsorption Kinetics of Au (III) onto CLPC Adsorbent ............................................. 46

2.3.6 Crystalline Structure of Cellulose .................................................................................... 49

2.3.7 Analysis of Cross-linked Polysaccharide Adsorbents After Au(III) Adsorption .. 50

2.3.8 Measurement of Fourier Transformed Infrared Spectra ........................................... 53

2.3.9 Proposed Adsorption-Reduction Reaction Mechanism .............................................. 55

2.4 CONCLUSION ................................................................................................................................... 58


iv

CHAPTER 3

Development of Bio-adsorbent for Selective Recovery of Au(III) from Water

Using Cotton Cellulose Treated with Concentrated Sulphuric Acid 61-90

3.1 INTRODUCTION ............................................................................................................................. 62

3.2 EXPERIMENTAL PROCEDURE .................................................................................................... 64

3.2.1 Materials and Methods .................................................................................................64

3.2.2 Preparation of Adsorption Gel from Cotton .............................................................64

3.2.3 Instrumental Analysis ...................................................................................................65

3.2.4 Measurement of the Degree of Crystallinity of Cotton Cellulose .............................65

3.2.5 Adsorption Test of Various Metals Ions in Batch-wise System from Acidic

Chloride Media........................................................................................................66

3.2.6 Thermo Gravimetric Analysis (TGA) .............................................................................. 67

3.3 RESULTS AND DISCUSSION ........................................................................................................ 68

3.3.1 Properties of Cotton Gel after Modification with Concentrated Sulfuric Acid ..... 68

3.3.1.1 Percentage yield and TOC leak test ................................................................ 68

3.3.1.2 Studies of crystalline structure of cotton cellulose ...................................... 68

3.3.2 Effect of Hydrochloric Acid Concentration on the Adsorption of Metal Ions ...... 69

3.3.3 Adsorption Kinetics Behavior of Au(III) onto Cotton Adsorbent ............................ 70

3.3.4 Adsorption Isotherms and Thermodynamic Investigation ........................................ 72

3.3.5 Solid State Analysis of Cotton Gel after Contacting with Au(III) Solution ........... 77

3.3.6 Fourier Transformed Infrared Spectroscopy Studies ................................................. 78

v

3.3.7 Thermo-gravimetric Analysis for the Recovery of Au(III) in Its Elemental

Form .................................................................................................................................. 80

3.3.8 Mechanism of Au(III) Adsorption-Reduction onto Cotton Adsorbent ............. 81

3.3.9 Recovery of Gold from the Leach Liquor of Spent Mobile Phones ..................... 83

3.4 CONCLUSIONS ................................................................................................................................ 84


Chapter 4

An Assessment of Gold Recovery Processes Using Cross-linked Paper Gel

91-119

4.1 INTRODUCTION ............................................................................................................................ 91

4.2 EXPERIMENTAL ........................................................................................................................... 93

4.2.1 Materials and Method ................................................................................................. 93

4.2.2 Preparation of Adsorption Gel ................................................................................ 94

4.2.3 Measurement and Analysis ....................................................................................... 95

4.2.4 Batch-wise Adsorption Tests ................................................................................... 96

4.2.5 Thermo Gravimetric Analysis (TGA) .................................................................... 97

4.3 RESULTS AND DISCUSSION ....................................................................................................... 97

4.3.1 Product yield and TOC leak of CLP gel ................................................................... 97

4.3.2 Effect of Hydrochloric Acid Concentration on the Adsorption of Metal Ions . 98

4.3.3 Adsorption Kinetic Studies ........................................................................................... 99

vi

4.3.4 Adsorption Isotherms and Thermodynamic Investigation ................................ 103

4.3.5 Solid-State Analysis of Gel After the Adsorption of Au(III) .............................. 107

4.3.6 Fourier Transformed Infrared Spectra ................................................................... 110

4.3.7 Recovery of Gold from Gold-loaded Adsorbent by Incineration ....................... 113

4.3.8 Proposed Mechanism for Adsorption Followed by Reduction of Au(III) ....... 114

3.4 CONCLUSIONS ............................................................................................................................... 117

REFERENCES CITED ......................................................................................................................... 118

Chapter 5

Adsorptive Recovery of Trace Concentration of Au(I) from Model Solution in

Sodium Hypochlorite Media 120-143

5.1 INTRODUCTION ............................................................................................................................. 120

5.2 EXPERIMENTAL PROCEDURE .................................................................................................. 124

5.2.1 Preparation of Concentrated H2SO4 Cross-linked Pure Cellulose Gel ................... 124

5.2.2 Chemicals and Material Used ........................................................................................... 124

5.2.3 Batch-wise Studies of Au Adsorption in NaClO Media .............................................. 125

5.3 RESULTS AND DISCUSSION ....................................................................................................... 126

5.3.1 Elemental Analysis of Gel Before and After Adsorption of Au by EDX

..................................................................................................................................................... 126

5.3.2 Recovery of Au(I) without NaClO and HCl solution ................................................... 127

vii

5.3.3 Effect of NaClO Concentration and Adsorption Mechanism of Au(I) After

Oxidation ................................................................................................................................... 128

5.3.4 Kinetic Experiment ............................................................................................................ 130

5.3.5 Effect of Adsorbent Dose .................................................................................................... 131

5.3.6 Recovery Attempt of Au(I) from Aqua Regia Solution .............................................. 133

5.3.7 Adsorption Isotherm Test of Au(I) After Oxidation at Different Temperature ... 135

5.3.7 XRD Measurement of Au(I) Loaded Cellulose Gel .................................................... 137

5.3.8 Optical Microscope Image of Filtered Cake of Cellulose Gel After Au Adsorption

.................................................................................................................................................. 138

5.3.9 NaClO in Hydrochloric Acid ............................................................................................. 139

5.4 CONCLUSIONS ..................................................................................................................... 141

REFERENCES CITED ...................................................................................................................... 142

PART -II

CHAPTER 6

Leaching of Gold from Mongolian Gold Ore by Acidic Thiourea Solution

145-158

6.1 INTRODUCTION ............................................................................................................................. 145


viii

6.3 RESULTS AND DISCUSSION ...................................................................................................... 147

6.3.1 Distribution of Particle Sizes ........................................................................................... 147

6.3.2 Effect of Leaching Solution ............................................................................................. 150

6.3.3 Effect of Liquid-solid Ratio for the Leaching of Gold and Platinum ..................... 151

6.3.4 Effect of Contact Time for the Leaching of Gold ....................................................... 152

6.3.5 Elemental Analysis of the Mongolian Gold Ore Sample Before and After

Leaching .................................................................................................................................... 153

6.3.6 Recovery of Gold from Leach Liquor Prepared in Acidic Thiourea Solution ... 155

6.3.7 Solid State Analysis ........................................................................................................... 156

6.4 CONCLUSIONS ............................................................................................................................ 157

REFERENCES CITED ...................................................................................................................... 158

CHAPTER 7

Recovery of Gold and Silver from Scraps of Plasma T.V. Monitor …159-177

7.1 INTRODUCTION ............................................................................................................................. 159

7.1.1 Advantages of Thiourea Leaching Over Cyanide Leaching ...................................... 161


7.3 RESULTS AND DISCUSSION ...................................................................................................... 163

7.3.1 Evaluation of Suitable Leaching Agents ........................................................................ 164

7.3.2 Optimization of Leaching Condition from Plasma T.V. Monitor .........................164

ix

7.3.2.1 Effect of thiourea concentration for leaching of silver .........................164

7.3.2.2 Effect of sulfuric acid concentration for leaching of silver .................165

7.3.3 Acidic Thiourea Leaching ............................................................................................... 166

7.3.3.1 Effect of liquid/solid ratio ............................................................................. 166

7.3.3.2 Effect of temperature for leaching of silver............................................... 168

7.3.4 Recovery of Gold and Silver from Leached Liquor Prepared from Acidic

Thiourea ............................................................................................................................ 168

7.3.4.1 Recovery of gold using cross-linked cotton gel ......................................168

7.3.4.2 Recovery of silver by cementation with zinc powder ............................170

7.3.5 Characterization of the Gold-loaded Cotton and Cemented Silver ...................... 172

7.4 CONCLUSION .............................................................................................................................. 175

REFERENCES ..................................................................................................................................... 176

CHAPTER 8

Concluding Remarks and Future Outlook 178-185

8.1 CONCLUDING REMARKS ......................................................................................................... 178

8.2 FUTURE OUTLOOK .................................................................................................................... 185

Appendices ............................................................................................................................... 186-195

Appendix I .......................................................................................................................................... 187

x

List of Published Papers ................................................................................................187

Appendix II ........................................................................................................................................ 190

Recently Submitted Manuscript and Manuscript in Preparation ............................190

Appendix III ...................................................................................................................................... 191

Participation at Different Scientific Meetings or Conferences ..................................191

I

List of Figures

Figures Page No

Figure 1.1 Gold deposited on some rocks (a) and (b), Gold present in discarded electronic parts

of Mobile phones (c) ........................................................................................................................7

Figure 1.2 Chemical speciation diagrams of Au, Pt and Pd in acidic chloride media ....................9

Figure 1.3 Molecular structure of cellulose .................................................................................18

Figure 2.1 Effect of hydrochloric acid concentration for the adsorption of various metal ions

onto CLPC adsorbent. Conditions: volume of solution = 10 cm3, concentration of metals = 0.1

mmol dm-3

, dry weight of gel = 10 mg, shaking time = 48 h, temperature = 303 K .....................38

Figure 2.2 SEM micrographs of (a) cross-linked cellulose, (b) cellulose after gold adsorption,

(c) cross-linked dextran, and (d) dextran after gold adsorption taken at 20 kV acceleration energy,

at 1500x magnification, scale = 5 µm, respectively .....................................................................39

Figure 2.3 Adsorption isotherms of Au(III) on cross-linked cellulose, cross-linked dextran,

cross-linked alginic acid, and cross-linked pectic acid, adsorbents (a) Experimental plots and (b)

Langmuir plots. Conditions: volume of the feed solution = 10 cm3, weight the dried gel added =

10 mg, shaking time = 150 h, [HCl] = 0.1 mol dm-3

and temperature = 303 K ............................41

Figure 2.4 Effect of temperature for the adsorption of Au(III) onto CLPC adsorbents (a)

Experimental plot, (b) Langmuir plot and (c) van’t Hoff plot. Conditions: dry weight of the gel =

10 mg, shaking time = 150 h, volume of the test solution = 10 cm3, [HCl] = 0.1 mol dm

-3 .........46

II

Figure 2.5 Adsorption kinetics of Au(III) onto CLPC adsorbent at different temperature (a)

experimental plot, (b) pseudo-first order plot, and (c) Arrhenius plot. Conditions: weight of the

dry gel added = 200 mg, volume of the feed solution = 200 cm3, [Au(III)] = 2 mmol dm

-3, [HCl]

= 0.1 mol dm-3

................................................................................................................................49

Figure 2.6 X-ray diffraction pattern of cellulose (multiple peaks) and amorphous cellulose

(single smooth peak) generated with concentrated sulphuric acid (96%) treatment. X-axis: Bragg

angle (2θ). I002 represents the maximum intensity at 2θ = 22.5º. Iam shows the minimum intensity

at 2θ = 18º used to calculate crystallinity in the peak height method ...........................................50

Figure 2.7 XRD patterns of gold-loaded cross-linked cellulose, cross-linked dextran, cross-

linked alginic acid, and cross-linked pectic acid adsorbents .........................................................52

Figure 2.8 Optical microscope photographs of gold-loaded cross-linked (a) cellulose (b) dextran

(c) alginic acid and (d) pectic acid, adsorbents ..............................................................................53

Figure 2.9 FT-IR spectra of (a) crude, (b) cross-linked and (c) gold-loaded adsorbents of (A)

cellulose, and (B) dextran ..............................................................................................................54

Figure 3.1 Cotton used for preparing the adsorbent......................................................................65

Figure 3.2 X-ray diffraction pattern of raw cotton (black, multiple peaks) and cotton gel (red,

single broad peak) prepared by using concentrated sulphuric acid (96%). ..................................68

Figure 3.3 Adsorption of metals ions on cotton gel as a function of hydrochloric acid

concentration. Conditions: weight of gel = 10 mg, feed solution = 10 cm3, concentration of

metals = 0.2 mmol dm-3

, shaking time = 24 h, temperature = 303 K ............................................69

III

Figure 3.4 Adsorption rate of Au(III) by the cotton gel at different temperatures. (a)

experimental plot, (b) pseudo first order plot and (c) Arrhenius plot. Conditions: weight of the

dry gel = 200 mg, volume of the solution = 200 cm3, concentration of Au(III) = 2 mmol dm

-3,

[HCl] = 0.1 m mol dm-3

.................................................................................................................72

Figure 3.5 Adsorption isotherms of Au(III) onto cotton gel at different temperature. (a)

Experimental plot, (b) Langmuir plot and (c) van’t Hoff plot. Conditions: weight of dry gel = 10

mg, volume of solution = 10 cm3, shaking time = 96 h, [HCl] = 0.1 mmol dm

-3 ..........................76

Figure 3.6 XRD patterns of cotton gel after the adsorption of Au(III) by CLC gel .....................77

Figure 3.7 Optical microscope photograph of gold aggregates (a) floating on the surface of

sample solution and (b) dried gel, after adsorption of Au(III) .....................................................78

Figure 3.8 FT-IR spectra of (a) raw cotton (b) cross-linked cotton (CLC) gel and (c) Au(III)-

loaded cross-linked cotton (Au-CLC) gel ......................................................................................79

Figure 3.9 Thermo-gravimetric curves of (a) gold-loaded cotton gel, (b) cotton gel before the

adsorption of Au(III) and (c) image of optical microscope of the gold-loaded cross-linked cotton

gel after incineration ......................................................................................................................80

Figure 3.10 Effect of solid/liquid ratio on the % adsorption of various metals from leach liquor

of chlorine containing hydrochloric acid solution for circuit board of spent of mobile phones

using cross-linked cotton gel. Condition: volume of solution = 10 cm3, shaking time = 48 h,

shaking speed = 150 rpm, temperature = 303 K ............................................................................84

Figure 4.1 Structure of paper cellulose .........................................................................................94

IV

Figure 4.2 Adsorption of various metals by cross-linked paper gel at varying hydrochloric acid

concentrations. Conditions: metal concentrations = 0.2 mmol dm-3

, volume of solution = 10 cm3,

weight of gel = 10 mg, shaking time = 24 h, temperature = 303 K ...............................................99

Figure 4.3 Adsorption kinetics of Au(III) by concentrated sulphuric acid cross-linked paper gel

at different temperature (a) Experimental plot, (b) Pseudo first order plot, (c) Arrhenius plot.

Conditions: volume of solution = 200 cm3, weight of the dry gel added = 200 mg, concentration

of Au (III) = 2 mmol dm-3

, [HCl] = 0.1 mol dm-3

........................................................................102

Figure 4.4 Adsorption isotherms of Au(III) on cross-linked paper gel at different temperature (a)



-3 .........106

Figure 4.5 Optical microscope photograph of gold-loaded cross-linked paper gel ....................108

Figure 4.6 X-ray diffraction powder patterns of Au(III)-loaded CLP showing the presence of

Au(0) ...........................................................................................................................................109

Figure 4.7 SEM micrographs of cross-linked paper gel (a) before and (b) after the adsorption of

Au(III). 290 x magnification, acceleration voltage = 20 kV, scale = 30 µm ...............................110

Figure 4.8 FT-IR spectra of (a) crude, (b) cross-linked and (c) gold-loaded paper gel .............112

Figure 4.9 Thermo-gravimetric analysis of cross-linked paper gel (a) before gold-loading, (b)

after gold-loading and (c) recovered gold after incineration .......................................................114

Figure 4.10 Inferred adsorption-reduction mechanism for Au(III) using cross-linked paper gel

......................................................................................................................................................116

Figure 5.1 Energy dispersive X-ray spectra of (a) cross-linked cellulose gel, and (b) cross-linked

cellulose gel after Au-adsorbed ...................................................................................................126

V

Figure 5.2 Effect of solid/liquid ratio for the recovery of Au(I) from Na3(Au(I)SO3)2 solution

prepared in water without using NaClO and HCl. Condition: [Au(I)] = 0.3 mmol dm-3

, volume of

solution = 10 cm3, temperature = 303 K, shaking time = 30 h, shaking speed = 150 rpm (Initial

pH = 7.56 itself) ..........................................................................................................................128

Figure 5.3 Effect of NaClO concentrations for the adsorption of metal ions. Conditions: metal

concentrations = 0.2 mmol dm-3

, [HCl] = 0.1 mol dm-3

, shaking time = 24 h, shaking speed = 150

rpm at 303 K, volume of solution = 10 cm3, amount of gel = 10 mg ..........................................129

Figure 5.4 UV-visible spectra of Au(I), Au(III) and NaClO treated Au(I) in the presence of HCl

showing the oxidation of Au(I) to Au(III) ..................................................................................130

Figure 5.5 Effect of contact time for the adsorption of Au(I) onto cross linked cellulose gel.

Conditions: [Au(I)] = 0.5 mmol dm-3

, volume of the solution = 250 cm3, weight of the gel = 250

mg, [NaClO] = 1 mol dm-3

, [HCl] = 0.1 mol dm-3

, temperature = 303 K ..................................131

Figure 5.6 Effect of adsorbent dose for the recovery of gold using cross-linked cellulose gel.

Condition: [Au(I)] = 0.3 mmol dm-3

prepared in water, [NaClO] = 0.1 mol dm-3

, pH = 3,

temperature = 303 K, shaking time = 24 h, volume of solution = 10 cm3, shaking = 150 rpm ...132

Figure 5.7 Adsorption test of Au(I) as a function of pH. Condition: Initial [Au (I)] = 0.5 mmol

dm-3

, [NaClO] = 1 mol dm-3

, Shaking time = 24 h, shaking speed = 150 rpm at 303 K ...........133

Figure 5.8 Adsorption test of Au(I) from aqua regia after adjusting pH to 4 by adding enough

amount of NaOH. Condition: initial [Au(I)] = 0.5 mmol dm-3

, volume of solution = 10 cm

3, initial

pH = 4, shaking time = 24 h, shaking speed = 150 rpm, ORP value after adjusting pH to 4 = 676.8

mV................................................................................................................................................134

VI

Figure 5.9 Adsorption isotherm of Au(I) using CLPC gel. (a) Experimental plot, (b) Langmuir plot,

and (c) van’t Hoff plot. Condition: weight of gel = 10 mg, volume of solution = 10 cm3, [NaClO] =

1 mol dm-3

, [HCl] = 0.1 mol dm-3

, shaking time = 96 h, shaking speed = 150 rpm ....................137

Figure 5.10 XRD patterns of the cross-linked cellulose gel after contacting with Au(I) solution

......................................................................................................................................................138

Figure 5.11 Optical microscopic images of elemental gold observed onto the cellulose gel (a)

after Au(I) adsorption, and (b) & (c) during Au(I) adsorption, volume of the solution = 10 cm3,

weight of adsorbent = 10 mg, shaking time for (b) = 96 h (2 mmol dm-3

) and for (c) = 24 h (0.2

mmol dm-3

) ..................................................................................................................................139

Figure 5.12 Ionization curve of HOCl as a function of pH.........................................................140

Figure 6.1 Particle size distribution of Mongolian gold ore sample ..........................................148

Figure 6.2 Cycle test for the leaching of Au and Pt from Mongolian gold ore sample using acidic

thiourea solution (0.5 mol dm-3

TU and 0.1 mol dm-3

H2SO4). Shaking time = 1 h, shaking speed

= 800 rpm, temperature = 70 °C, weight of gold ore sample = 5 g, volume of solution = 60 cm3

......................................................................................................................................................149

Figure 6.3 Effect of leaching solution for gold at different conditions. Shaking time = 24 h,

shaking speed = 150 rpm, at 303 K..............................................................................................151

Figure 6.4 Effect of liquid/solid ratio for the leaching of gold and platinum from Mongolian gold

ore at 0.1 mol dm-3

TU, 0.05 mol dm-3

H2SO4 for 24 h at 30°C shaking speed = 150 rpm, sample

powder = 1 g ..............................................................................................................................152

VII

Figure 6.5 Effect of contact time for the leaching of gold at 30 cm3

g-1

of solid/liquid ratio using

0.1 mol dm-3

TU and 0.05 mol dm-3

H2SO4 at different temperature at shaking speed of 300 rpm

......................................................................................................................................................153

Figure 6.6 Energy dispersive X-ray spectra of (a) Mongolian gold ore soil sample before and (b)

Mongolian gold ore soil sample after leaching with acidic thiourea ...........................................155

Figure 6.7 Effect of solid/liquid ratio for the adsorption of gold using cotton gel from real leach

liquors of Mongolian gold ore soil sample prepared from acidic thiourea solution. Volume of the

liquor = 10 cm3, temperature = 303 K, shaking time = 24 h, at 150 rpm ...................................156

Figure 6.8 Solid state analysis of gold loaded CLC adsorbent (a) digital photograph, and (b)

XRD pattern .................................................................................................................................157

Figure 7.1 Leaching behavior of silver at different thiourea concentration as a function of

liquid/solid ratio. Conditions: weight of the powder used = 100 mg, sulphuric acid = 0.05 mol

dm-3

, shaking time = 24 h, temperature = 303 K .........................................................................166

Figure 7.2 Leaching behavior of silver as a function of acid concentration. Conditions: weight of

the powder used = 100 mg, [TU] = 0.1 mol dm-3

, liquid/solid ratio = 500 cm3

g-1

, shaking time =

24 h, temperature = 303 K, shaking speed = 150 rpm .................................................................167

Figure 7.3 Leaching behaviors of various metals ions as a function of Liquid/solid ratio. [TU] =

0.1 mol dm-3

, weight of powder leachate = 100 mg, [sulphuric acid] = 0.05 mol dm-3

,

temperature = 303 K, shaking time = 24 h, shaking speed = 150 rpm ........................................168

Figure 7.4 Leaching kinetics of silver at different temperatures liquid/solid ratio = 200 cm3 g

-1,

[TU] = 0.1 mol dm-3

and [H2SO4] = 0.05 mol dm-3

, shaking speed = 600 rpm ...........................169

VIII

Figure 7.5 Effect of solid/liquid ratio for the adsorption of gold from real leach liquors of scraps

of plasma T.V. monitor. Volume of the liquor = 10 cm3, temperature = 303 K, shaking time = 24

h, at 150 rpm ...............................................................................................................................171

Figure 7.6 Recovery of metallic silver by cementation with zinc powder at varying solid/liquid

ratio. Conditions: volume of acidothiourea leach liquors = 10 cm3, temperature = 303 K, shaking

time = 24 h, shaking speed = 150 rpm .........................................................................................173

Figure 7.7 XRD patterns for metallic silver cemented on zinc powder .....................................174

Figure 7.8 Optical microscope image of metallic silver cemented on zinc powder (a) 150x

magnifications, (b) 300x magnifications and (c) digital photograph of CLC gel after the

treatment with leach liquor and (d) XRD pattern of gold-loaded cotton gel ...............................175

IX

List of Tables

Tables Page No

Table 1.1 Stability constants data for gold, platinum and palladium ................................................ 8

Table 1.2 Common chloro-species of precious metals ...................................................................... 15

Table 2.1 Langmuir isotherm parameters for Au(III) onto cross-linked polysaccharide gels ......42

Table 2.2 Comparison of maximum adsorption capacities for Au(III) on different adsorbents

.................................................................................................................................................................... 43

Table 2.3 Thermodynamic parameters for Au (III) by cross-linked cellulose gel .......................... 44

Table 3.1 Thermodynamic parameters for the adsorption of Au(III) on acid treated cotton gel . 73

Table 3.2 Comparative account of maximum adsorption capacity for Au(III) among various

adsorbents ................................................................................................................................................ .74

Table 4.1 First order rate constants and corresponding correlation coefficient for the adsorption

of Au(III) on cross-linked paper gel at different temperatures ........................................................ 101

Table 4.2 Thermodynamic parameters for Au (III) by cross-linked paper gel.............................. 105

Table 4.3 Comparison of maximum adsorption capacities for Au(III) with reported adsorbent

together with CLP gel investigated in this study ............................................................................... 107

Table 5.1 Thermodynamic Parameters for Au(I) by cross-linked cellulose gel ........................... 136

Table 6.1 Quantitative analysis of metal ions in Mongolian gold ore by acidic thiourea solution

.................................................................................................................................................................. 148

Table 7.1 Quantitative analysis of scraps of plasma T.V. monitor ................................................. 165

X

List of Schemes

Schemes Page No

Scheme 2.1 Inferred example of synthetic route of cross-linked cellulose and its adsorption-

reduction mechanis ..................................................................................................................57

Scheme 3.1 Synthetic route of cross-linked cotton gel and mechanism of Au(III) adsorption

followed by reduction ...................................................................................... ……………... .. ….82

Scheme 4.1 Preparation of cross-linked paper (CLP) adsorbent ............................................... ..95

XI

Acknowledgements

Completion of this doctoral dissertation was possible with the support of several people. I would

like to express my sincere gratitude to all of them.

First and foremost I would like to express my sincere indebtedness and deepest gratitude to the

Prof. Emirates Katsutoshi Inoue for providing me with the opportunity to complete my Ph.D.

thesis at Saga University in Chemical Engineering Laboratory. I extremely grateful for his

invaluable suggestion, scholarly inputs and consistent encouragement I received throughout the

period of my research work. This accomplishment was possible only because of the

unconditional support provided by Prof. Inoue. In this respect, I feel very lucky to work under

his great guidance.

It is my great pleasure to express the highly appreciated words to Prof. Keisuke Ohto,

Department of Energy and Materials Science, Saga University, for accepting me as a graduate

student under his supervision. I am kindly indebted to Prof. Ohto for his encouraging and

supportive suggestions, and greatly thankful for his all the academic supports and all the

facilities provided to me throughout my research work.

I would like to extend my sincere and cordial thankful to the Associate Prof. Hidetaka Kawakita

for his great help and support, invaluable suggestions, and many and uncountable

encouragements throughout my research period at Saga University. It is my great pleasure to

acknowledge Assoc. Prof. Hiroyuki Harada (Hiroshima Prefectural University) for providing the

facilities for TOC measurements. I am thankful to the Associate Prof. Shintaro Morisada for his

kind hospitality and cooperation.

I thank all the Japanese friends in our Laboratory for their help and fun time during various

moments (since 2009 to now). All the happy and enjoyable times with them are always truly

memorable for me.

I gratefully acknowledge the funding sources that made my Ph.D. work possible to the Saga

University International Student Center for providing me the financial inputs, JASSO and Saga

City Scholarship.

XII

In this very important and precious time, I would like to express my sincere thanks to all the

teachers and friends since from my childhood for their moral support and motivation. I am also

indebted and feel very fortunate to express my wishes to the Prof. Ram Prasad Chaudhary (M.Sc.

Supervisor) at Central Department of Botany, Tribhuvan University, Nepal for his inspiring

guidance.

I would like to acknowledge the seniors and friends from different countries in our Lab for their

cooperative behavior. I am thankful to all Nepalese of Saga University for the helps joyful time

all the years that we enjoyed together, thank you all.

I would like to express my indebtedness to my parents, brothers and sister for always believing

in me and teaching me that any goal I set forth is attainable. Their support, love and prayers

helped me through the time of this degree and I can never thank them enough.

Finally, and most importantly, I would like to give my special thanks to my beloved husband Dr.

Hari Paudyal who supported me in every possible way to see the completion of this work. The

thesis would not have come to a successful completion, without his immense helps and supports.

He has been a constant source of extraordinary strength and inspiration for me. It was only his

determination, constant encouragement, positive stimulations and decision making capability

that ultimately enabled me to complete this work. I am blessed by my daughter (Aayushma) just

one year before of my Ph.D completion and tremendously happy and getting more source of

energy to do more effort in my life.

Bimala PangeniBimala PangeniBimala PangeniBimala Pangeni

March 2014March 2014March 2014March 2014

XIII

ABSTRACT

DEVELOPMENT OF THE LOW COST ADSORBENTS FROM POLYSACCHARIDES-

BASED BIOMASS FOR THE RECOVERY OF GOLD

By

Bimala Pangeni

Chairperson of Supervisory Committee: Professor Keisuke Ohto

Department of Energy and Material Science, Saga University

In the broad field of research nature, there are numerous material originated from

biomass and have been used as the adsorbent for separating and recovering the precious metals

from various sources. Lots of works have been conducted for the recovery of precious metals by

adsorption process. However, several researchers have still facing the disadvantages of using

biomass based adsorbent for precious metals recovery in terms of the selectiveness and

adsorption capacity. In order to strength the properties of biomass-based adsorbents, chemical

modification of polymeric matrices in biomass is one of the important and required things to

develop the suitable types of adsorbents by creating the active sites towards the targeted metal

ions. Polysaccharide especially cellulose is most abundant material in the earth which is easily

available low cost and can be easily modified into various forms to extend their capability. The

main objective of the present research investigation is to develop an efficient and low cost

adsorption material for the perspective recovery of precious metals (especially gold) from

polysaccharide like cellulose and to formulate the suitable mechanisms from aqueous as well as

XIV

real industrial effluents. Thus, in the present work, the some commercial polysaccharide such a

pure cellulose powder was firstly used to prepare the adsorbent by cross-linking with

concentrated sulphuric acid for adsorbing precious metals and base metals from mixture and

individual solution then it was compared with other polysaccharide adsorbent prepared from

exactly the same method of cross-linking. The potentiality of these adsorbents for adsorptive

recovery of precious metals in hydrochloric acid media at varying concentration from aqueous

solution was evaluated. The adsorbents thus prepared by cross-linking with concentrated

sulphuric acid were found to be extremely selective and highly adsorb gold from acidic chloride

media which contain other precious metals such as Pt(IV) and Pd(II) and usually co-exist base

metals. Based on these outcomes, we had elaborated or extended our technology to the cotton

and paper biomass. These biomaterials also exhibited tremendously selectiveness and high

adsorption capacity toward the Au(III) adsorption from acidic chloride media in presence of

other precious and base metals. These all adsorption behavior of polysaccharide-based adsorbent

showed that the noble metals Au(III) can be separated with high efficiently from aqueous

solution in acidic chloride media and as compared to other bio-adsorbents, the bio-adsorbent on

biomass and paper biomass exhibited higher adsorption capacity towards Au(III). In all cases the

adsorbed Au(III) was reduced to its elemental form which was confirmed XRD and EDX

spectroscopy. Thus adsorption followed by reduction process was favored in the present

investigation. The recovery of zero-valent gold from gold loaded adsorbents can easily be

achieved by simple incineration process at high temperature.

In addition to the successful adsorption of Au(III) from aqueous as well as from real

waste solution by using various polysaccharide-based adsorbents from acidic chloride media for

its recovery in its metallic form by incineration process. Further, adsorption tests of Au(I) from

XV

sodium gold sulphite solution using the cross-linked polysaccharide adsorbent was conducted in

sodium hypochlorite media to get some idea for the recovery of Au(I) from cyanide solution as

alternative method. Also, the adsorption followed by reduction to the metallic gold from Au(I)

solution was successful by using cross-linked cellulose gel. The successful recovery of gold from

aqua regia after adjusting pH to 4 was also promising results by using cross-linked cellulose gel.

Furthermore highly toxic cyanide solution was traditionally used for gold extraction from

waste electronic appliances and ore sample. Our study also focused on the investigation of non

toxic, low cost and highly efficient leaching solution for gold and other precious metals. Highly

efficient leaching solution (acidified thiourea solution) was investigated for quantitative leaching

of gold and silver from their primary as well as secondary sources. Prior to recovery step,

leaching of precious metals like gold was carried out from primary source (Mongolian gold ore)

and from secondary source (Scraps of plasma T.V. monitor) by acidic thiourea solution then it

was followed by their recovery using adsorption onto cross-linked cotton gel for gold and zinc

cementation for silver. Hence, based on the results observed in the present research work, the

novel use of polysaccharides-based waste biomass as well as commercially available powder

form after simple chemical modification with concentrated sulphuric acid can be converted into

an effective and suitable adsorption materials for the selective adsorption of Au(III) and its

recovery in metallic form by incineration process from aqueous as well as from some industrial

waste effluents was conducted.

1

CHAPTER 1CHAPTER 1CHAPTER 1CHAPTER 1

Research Background and Objectives of the Present Research Work

1.1 RESEARCH BACKGROUND

The biosphere in principle contains the essential minerals necessary for growth, survival

and multiplication of the diverse spectra of living organisms, as well as all of the remaining

naturally occurring elements of the periodic table1. In the present research investigation, the

concept of selective element accumulation characteristically displayed by individual biomass

waste from almost the major polysaccharides sources is emphasized. In particular, some

polysaccharides-based biomass after some chemical modification observed to accumulate

relatively large concentrations of precious metals including gold, silver and so on. These

accumulator or adsorbents of polymeric biomass may indeed be detected in various experimental

conditions in laboratory scale from aqueous as well as from actual solution for recovering

precious metals especially, gold and silver.

1.1.1 Precious Metals

A precious metal is a rare metallic chemical element of high, durable economic values

and plays unique role in the society, least reactive towards atoms or molecules at the interface

with a gas or a liquid but forms stable alloys with many other metals or element.

2

Precious metals are naturally occurring metallic elements which showed less reactive

among other metallic element which are usually ductile, high lusture and high economic value.

The best-known precious metals are platinum group metals (PGM) and coinage metals, gold and

silver. Moreover, the metal is considered to be precious if it is rare and high market value with

excessive demand. Precious metals in bulk form are known as bullion and are traded on

commodity markets which are mainly valued by its mass and purity2. Investigation of new cost

effective mining and processing in addition to the more economic method for the recycling of

precious metal from wasted scraps of electrical devices and waste solution of metal plating bath

is necessary to diminish the market price.

The great scarcity of precious metals is due to their occurrence in trace concentration in

their ore and complicated process for their extraction and purification. These metals have wide

industrial application, they are better known for their uses in art, jewelers and coinage. Thus, the

term “precious” reflects their economic value as well as their rare occurrence. The precious

metals do not have tendency to form oxides under standard condition with the exception of

osmium which smells even at room temperature due to the formation of OsO4. Silver and gold

are not susceptible to oxidation by hydrogen ions under the standard condition, and this noble

character accounts for their use, together with platinum, in jewelers and ornaments3. Because of

their resistance to corrosion and oxidation, high melting point, electrical conductivity, catalytic

activity and biological inertness, these elements have wide applications in chemical, electrical

and electronic glass, medicine and automotive industries3.

3

1.1.2 Platinum Group Metals (PGM)

The “platinum group metals” (PGM) also called platinoid is a term used to collectively

refer to six metallic elements clustered together in the periodic table. These elements are all

transition metals, lying in the d-block (groups 8, 9 and 10, period 5 and 6). The platinum group

metal includes ruthenium, rhodium, palladium, osmium, iridium and platinum4. They have

different chemical behavior from the other elements of the group: Fe, Co and Ni. Their metallic

properties are such as to suggest classification in pairs: Ru/Os, Rh/Ir, Pd/Pt. Further, they are

usually sub-divided into two different groups: metals of the primary groups, which comprise

platinum and palladium, and the remaining elements of the group, which form the secondary

PGM. Such types of classification arise from the fact that with the conventional

hydrometallurgical method, the treatment of concentrates of precious metals is carried out by

means of lixiviation with aqua regia, which yields a first separation into a soluble part and an

insoluble residue, the soluble metals are gold and the metals of the primary platinum group and

the insoluble ones are silver and the secondary PGM.

1.1.3 Gold and Silver

The gold and silver are laid in group 11 in periodic table. The metal gold, silver and

copper have historically been used as components in alloys used to mint coins thus are also

known as coinage metals. The copper is 25th

most abundant element in the earth crust whereas

silver and gold are quite rare thus these are placed under the precious metals.

4

1.1.3.1 Silver

The silver is one of the precious metals with chemical symbol Ag and atomic number 47

which is 66th

most abundant metal by weight in earth crust. The silver occur naturally as sulphite

Ag2S (argentite) ore, chloride AgCl (horn silver) ore and its pure, free form (native silver) on

earth crust. Most silver is produced as a byproduct of copper, gold, lead, and zinc refining

whereas some amount of silver is extracted by Parke’s and cyanidation process. Mexico (17%),

USA (13%), Peru (11%), Canada (9%), Australia (9%), Soviet Union (7%), and Chile (7%) are

the main silver producing countries in the world5. The silver has long been valued as a precious

metal, used in currency coins, to make ornaments, jewelry, and high value tableware and as an

investment in the forms of coins and bullion. It is used industrially in electrical conductors,

mirrors and for the catalysis of chemical reactions. Its salts AgCl and AgBr had been widely

used in photographic emulsion whereas dilute concentration of AgNO3 is used as disinfectants.

1.1.3.2 Gold

The gold has been known and highly valued since prehistoric times. It may have been the

first metal used by humans and was valued for ornamentation6. The gold and the search for it

have been among the major factors in exploration, conquest, and growth of civilization7.

Archaeological studies have shown that the goldsmith's art dates from at least 4000 BC in

Mesopotamia and that it then spread throughout the whole group of ancient civilizations around

the eastern Mediterranean Sea, including Egypt. It also arose in the New World in the Pre-

Columbian cultures of Peru and Mexico, as well as in the Asian civilizations. Historically gold

has been found as lumps of metal in the ground called nuggets which was called gold rush in

USA. Gold is widely distributed throughout the world, normally in very low concentration and

5

generally in native form as metal. It is usually alloyed with silver and often contains small

amounts of copper.

The only compounds of gold found in nature are the tellurides, typically calaverite

(AuTe2), petzite ((AuAg)2Te), sylvanite ((AuAg)Te2), among others. Gold is found in native

form in both lode and alluvial deposits. The deposited gold in some rocks is demonstrated in

Figure 1.1 (a) and (b). It is probably the first metal known to and used by humans, since it

occurs in nearly pure form, and its color, weight and brightness make it easily distinguishable

from the sand and gravel with which it is usually associated. Lexicographers trace the word

“gold” to an old Gothic term meaning yellow; and the symbol Au is from Latin word “Aurum”.

The purity of gold is generally expressed in carat. Pure gold is 24 carat. South Africa (29%),

USA (14%), Australia (11%), Soviet Union (10%), Canada (7%) and China (6%) are major gold

producing countries in the world 5.

For the extraction of gold, the rocks containing trace amount of gold are first crushed into

powder form then gold is extracted either with mercury or sodium cyanide. If gold containing

rock powder is mixed with mercury then it forms the amalgam with gold that after distillation,

pure gold is recovered. In the cyanide process, gold ore was treated with sodium cyanide solution

in oxygenated environment to give soluble sodium argentocyanide complex. The gold is

precipitated from sodium argentocyanide by adding zinc powder. The small amount of gold is

used to make corrosion free electrical contact in different electrical appliances such as computer

board, mobile chief and other high tech equipments in addition to its application as coinage metal

or standard for monetary systems in many countries, jewelers or decorating purpose, plating for

coating space satellites, photography for toning the silver image and medicine as disodium

aurothiomalate for the treatment of arthritis and cancer.

6

1.2 WASTE ELECTRONIC DEVICES OR E-WASTE

Over the last decade the electronics industry has revolutionized the world: electrical and

electronic products have become ubiquitous of today's life on the earth. Without these products,

modern life would not be possible in industrialized and industrializing countries. These products

serve in such areas as medicine, mobility, education, health, food-supply, communication,

security, environmental protection and culture. Such appliances include many domestic devices

like refrigerators, washing machines, mobile phones, personal computers, printers, toys and TVs.

The amount of appliances put on market every year is increasing both in industrialized and

industrializing countries.

As a result of the overthrow of informatics technology, the production of electrical and

electronic equipments is steadily increasing in all around the world. Due to the economic growth,

technological innovation and business expansion of electrical and electronic equipments (EEE),

there is an inevitably uprising in the waste of electrical and electronic equipments that appears a

new and challenging environmental issue for the resource conservation and recycling of valuable

and recoverable metals products. Essential constituents of much EEE include precious metals

(gold, silver, and palladium) and special metals (indium, selenium, tellurium, tantalum, bismuth,

antimony)8, 9

. In fact, compared with natural gold ores, the gold content in electronic scarp is

significantly higher (10 - 10 kg gold per ton of electronic scrap materials as compared to 0.5 -

13.5 g gold per ton of natural gold ore10, 11

, creating an economic driving force for the recycling

of electronic waste.

7

In the recent situation, the crucial options for the treatment of e-waste are involved in

reuse, remanufacturing, and recycling, as well as incineration and land filling. Recycling is the

reprocessing in a production of waste materials for the original purpose or other purposes.

Recycling of e-waste involves the destruction of the end of life equipment in order to recover

materials. The hierarchy of the treatment of e-waste encourages reuse the whole equipment first,

remanufacturing, then recovery of materials by recycling techniques, and at last disposal by

incineration and land-filling. Some of the electronic scrap wastes with high precious metal

contents that are recovered as shown in Figure 1.1 (c).

Figure 1.1 Gold deposited on some rocks (a) and (b), Gold present in discarded electronic parts

of Mobile phones (c)

Thus, recycling of electronic waste is an important subject not only from the point of

waste treatment but also from the recovery aspect of valuable materials12-14

. There is a growing

anxiety about securing a stable supply, and therefore, the development of recycling technologies

is very important to utilize the resources efficiently 15

.

(b) (a)

(c)

8

1.3 CHLORIDE CHEMISTRY OF PRECIOUS METALS (GOLD, PLATINUM AND

PALLADIUM)

The stability constant data of the above mentioned precious metal ions in chloride media are

presented in Table 1.1. From these values, the stepwise formations of various metal chloro-

complexes are formulated. From this study, the most stable chloro-complexes of Au(III), Pt(IV)

and Pd(II) at wide range of hydrochloric acid (weak to strong media) are AuCl4-, PtCl6

2- and

PdCl42-

, respectively.

Table 1.1 Stability constants data for gold, platinum and palladium16

The chemical speciation diagrams of gold, platinum and palladium in acidic chloride

media are demonstrated in Figure 1.2. It is clear from these figures that, the Au(III), Pd(II)

and Pt(IV) ions exist as chloride complex anions over a wide range, consequently the anion

exchangers are suggested as the best sorbents for their recovery from chloride media.

Considering this fact, various adsorption gels, resins or solvent extraction reagents having

suitable functional groups have been developed and some of them are being utilized for the

recovery of these precious metals from hydrochloric acid media 17-21

.

Metal species Logarithmic values of stability constants

Au(III) K1 = 8.51, K2 = 8.06, K3 = 7.00, K4 = 6.07

Pt(IV) K5 = 3.70, K6 = 2.25, K1 to K4 are unavailable

Pd(II) K1 = 4.70, K2= 3.00, K3 = 2.60, K4 = 1.60

9

Figure 1.2 Chemical speciation diagrams of Au, Pt and Pd in acidic chloride media

100 Au3+

AuCl2+

AuCl4-

log[Cl-]

80

60

40

20

0

-12 -10 -8 -6 -4 -2

Au [%]

AuCl2+

AuCl3

80

60

40

20

0

-10 -8 -4 -2 0 -6 2

100 Pd [%]

log[Cl-]

Pd2+ PdCl4

2-

PdCl3-

PdCl+

PdCl2

log[Cl-]

100

80

60

40

20

0

-8 -6 -4 -2 0

Pt [%]

PtCl62-

PtCl5-

PtCl4

10

1.4 PRECIOUS METALS RECOVERY TECHNIQUES

The waste of electrical and electronic equipment (WEEE) with an annual growth rate of

about 3-5% is the fastest growing waste stream in municipal wastes22

. Heterogeneous and

complex natures of WEEE (i.e. metal diversity and metal-non-metal associations) are the main

obstacles for the recovery of metals from WEEE23

. The ever increasing demand for electronics,

fueled by a rising affluence in societies, coupled with the short lifetime of electronic devices, has

resulted in the generation of torrential electronic waste stream in our modern world24

. This

electronic waste stream presents a major disposal challenge as an electronic waste containing

toxic metals such as lead and mercury25

. In addition to its inherent toxicity, electronic waste also

contains significant amounts of precious metals such as gold. For the recovery of metal from

WEEE, various treatment options based on conventional mechanical, physical, pyrometallurgical

and hydrometallurgical processes are proposed. Conventional pyrometallurgical processes are

time and energy consuming, while hydrometallurgical processes are energy saving.

Pyrometallurgy processes is the combination of roasting, smelting, converting, fire-refining,

electrolytic refining and chemical refining26

. Nowadays, because of the depletion of high grade

ore resources and environmental concerns, significant proportions of metals are increasingly

recovered through hydrometallurgical treatments27

. However, in many cases, both

hydrometallurgical and pyrometallurgical treatments are employed or combined for

concentrating and separating precious metals especially from secondary sources which includes

the diversity of elements28

.

11

1.4.1 Recovery of Precious Metals by Hydrometallurgical Processes

With the advancement of technology, several methods of gold recovery have been

formulated. Nowadays, hydrometallurgical treatment are receiving more and more attention and

being employed substituting the pyrometallurgical treatment due to its low cost, ease of handling

flexibility, clean operating conditions and low emission of toxic gases etc. Hydrometallurgy is

one of the parts of extractive metallurgy involving the use of aqueous chemistry for recovery of

precious metals from primary (ore) as well as secondary resources (scraps of electrical and

electronic equipments etc.). The principle steps in hydrometallurgical process consist of a series

of acid or caustic leaching of solid materials. The solutions are then subjected to separation and

purification procedures such as precipitation of impurities, solvent extraction, adsorption and

ion-exchange to isolate and concentrate the targeted metal ions11

. There are many techniques in

hydrometallurgical processes among which chemical leaching for precious metals using cyanide,

halide, thiourea, and thiosulfate11, 29-33

are explained in detail in Section 1.4.1.1.

1.4.1.1 Leaching of gold

Leaching is the process of extracting the soluble constituent from the solid source by

means of a solvent, which is the initial step in a hydrometallurgical process. The most common

leaching agents used in recovery of precious metals include cyanide34

. The more restrictive

regulations for controlling the environmental pollution by cyanide bearing streams and materials

have given an impetus to the researches aiming the discovery of some new, more ecological

methods for gold dissolving35

and more efficient methods for gold separation and refinement36

.

The recent developments promote alternative approaches to the cyanidation process such as

thiourea, chlorine, bromine, iodine, thiosulfate, aqua regia leaching37

followed by recovery step.

12

Cyanide leaching: The cyanidation process has been used to leach gold over 100 years since it

was patented in 1888 by Mac Arthur and Forest brothers38

. Leaching by cyanide solutions in

aerated alkaline medium is the main process for gold extraction from ores. Gold cyanidation is

an electrochemical process where gold is oxidized and then complexed to the stable complex ion

[Au(CN)2]- and gold is recovered by adsorption onto activated carbon or by zinc precipitation.

The dissolution process of metal is given by chemical reaction as Eq. (1.1).

4Au + 8NaCN +O2 +2 H2O 4Na[Au(CN)2] + 4NaOH (1.1)

Thiourea leaching: This method was developed as a potential substitute for cyanidation (of gold

as well as silver). Lower toxicity of thiourea and greater rate of gold and silver dissolution

compared to cyanide give it an advantage in reaching commercial application before other non-

commercial lixiviants. The lower sensitivity to base metals or impurities renders possible the use

of this process in many refractory gold ores. Thiourea leaching has been tried on different

materials and ores with success. Recovery of gold from pyrite and chalcopyrite concentrate has

given 96 percent extraction. Extraction of 90 percent is reported on carbonaceous material. The

dissolved precious metals from pregnant solution can be recovered by suitable adsorptions such

as an activated carbon, strong acid cation exchangers and thio/resins or using electro-wining,

cementation and gaseous reduction. Most of the works done to recover gold with thiourea are on

the leaching part of the process. A few results are available on the recovery of gold and silver

from the pregnant solution shown in Eq. (1.2).

Au + 2CS(NH2)2 Au[CS(NH2)2]2+ +e

- (1.2)

13

1.4.1.2 Recovery of gold from Leachate

Exciting new developments are taking place in the extractive metallurgy of precious

metals which are based upon the adsorption of the metals or its complexes onto suitable

adsorbents and subsequent elution39

.

In order to recover the precious metals from the different solutions as mentioned earlier, a

variety of methods are found in literature. These methods include cementation40, 41

, solvent

extraction42-45

, adsorption on activated carbon46-48

, and ion exchange49-52

. However, these

techniques, in many cases, suffer from various drawbacks such as low selectivity for gold, low

efficiency in dilute solution, and high cost53

. Because of value and demand for precious metal

like gold in the modern age, the separation and recovery of gold from primary sources like gold

ore and secondary sources such as electronic scraps and waste gold plating solutions is an

important technology. Gold recovery has received significant attention because gold is present in

appreciable amount in electronic parts and plating materials54-56

. The increase in the industrial

demand for gold corresponds to the increase in the need for gold recycling. Generally, gold is

separated from industrial wastes by hydrometallurgical processes by using sodium cyanide

solution as a leaching solution, after which gold is recovered by means of cementation by adding

some reducing agent like zinc powder57

or adsorption on activated carbon.

The process known as carbon-in-pulp (CIP) or charcoal in pulp. CIP controls the gold

precipitation from the cyanide solution by use of activated charcoal or carbon58

. Activated

carbon can be manufactured from wood, nuts shells, coal, petroleum coke and a variety of

organic products. Coconut shell carbon is preferred because of its commendable durability and

high adsorption capability for gold and silver cyanide. Other modifications include Carbon-in-

leach (CIL) and Carbon-in-column (CIC)59

.

14

Ion-Exchange/Solvent Extraction is one of the emerging technologies which involve the

recovery of precious metals from pregnant solutions by ion-exchange resins using the resin-in-

column (RIC) technique. The process is similar to CIC. One major exception involves the

operation of the elution (stripping) stage which does not require elevated temperatures and

pressures for removal of precious metals from the loaded resins. Either strong or weak base

resins may be used. Stripping of the loaded gold values from the resin has proven to be difficult

and requires further research.

Because of the toxicity associated with cyanide and its ineffectiveness in refractory ores

and concentrates, cyanide leaching is not appreciated. Nowadays, several studies have been

focused on an alternative processes using non-cyanide lixiviants involving the total dissolution of

metals by chloride leaching where hydrochloric acid in the presence of chlorine gas is used for

increasing the oxidizing capability of the solution. The metal ions in the strong chloride medium

have resulted in the formation of water soluble chloro-anions of precious metals. For example,

gold leaching is expressed by Eq. 1.3 60

. The similar chloro-complexes of some precious metals

in such leach liquors are presented in Table 1.2.

2Au +2HCl + 3Cl2 2HAuCl4 (1.3)

15

Table 1.2 Common chloro-species of precious metals61

Metals Oxidation state Major chloro species Remarks

Gold(Au)

I AuCl2- Unstable

III AuCl4- Very stable

Palladium(Pd) II PdCl42-

Most common species

IV PdCl62-

Difficult conversion of

II to IV

Platinum(Pt) II PtCl42-

Slow conversion of IV

to II

IV PtCl62-

Most common species

and kinetically inert

The adsorptive recovery of these precious metals onto activated carbon and ion exchange

resins is effective only for the cases of high concentration of precious metals to be recovered

from the leached solution. Therefore, nowadays, various scientific research work are focused on

the utilization of biomass waste as adsorbent after immobilization of some functional group

through chemical modification for the recovery of trace concentration of targeted metal ions

from aqueous solution.

Adsorbents of interest in water treatment include activated carbon; ion exchange resins;

adsorbent resins; metal oxides, hydroxides, and carbonates; activated alumina; clays; and other

solids that are suspended in or in contact with water 62, 63

. Now, bio-sorption research has

revealed a strong potential for the recovery of gold. Biosorption can be defined as the uptake of

solutes by inactive/dead biological materials64, 65

. Several biomasses have been used for the

16

adsorptive recovery of gold including fungal biomass, tannin, alfalfa, various protein sources and

fruit wastes17-19, 66-69

. However, locally available and abundant low cost adsorbent materials are

still required in order to recover gold from particularly electronic and electroplating factories

wastewater70

. The cost effectiveness is the main features of bio-sorption. Additional advantages

of bio-sorption process are the biomaterials are easy to regenerate and it is possible to be reused71

.

However in the recent years, the companies mainly focus on the incineration of the used

adsorbent for the pure recovery targeted metal ions because the solid adsorbent after incineration

at high temperature has no significant effect to be added into the environment. Another

advantage of not doing elution by using some chemicals is that there may be the chance of

formation of new toxic compound during elution with some acid or base. Large amount of

eluting agent is required for treating and further treatment is also marked for some cost. So,

volume reduction of the used adsorbent is possible only from the incineration process therefore

in the present investigation the recovery of gold was achieved by incineration process at high

temperature.

1.5 POLYSACCHARIDES

In recent years, much attention has been focused on the use of various industrial wastes,

agricultural byproducts, and biological materials as metal salient. A number of studies have

suggested that bio-adsorbent provides a cost effective means of gold recovering from aqueous

solutions72

.

Biopolymers have recently received a great deal of attention due to the fact that they

represent renewable resources and are more environmentally friendly, biodegradable, and

biocompatible than conventional materials. Therefore, they have attracted significant attention in

17

recent years. Recently, numerous approaches have been studied for the development of cheaper

and more effective adsorbents containing natural polymers. Among these, polysaccharides such

as cellulose, dextran, pectic acid, alginic acid and their derivatives deserve particular attention in

addition to the materials that contains one or more above mentioned polysaccharides such as

cotton and paper. These biopolymers represent an interesting and attractive alternative as

adsorbents because of their particular structures, physico-chemical characteristics, chemical

stability, high reactivity and excellent selectivity towards aromatic compounds and metals,

resulting from the presence of chemical reactive groups (hydroxyl, acetamido or amino

functions) in polymer chains. Moreover, it is well known that polysaccharides resources have a

capacity to associate by physical and chemical interactions with a wide variety of molecules.

Hence, adsorption on polysaccharide derivatives can be a low-cost procedure in water

decontamination for extraction and separation of compounds, and a useful tool for protecting the

environment. Besides, the increasing numbers of publications on adsorption of toxic compounds

by these natural polymers show that there is a recent interest in the synthesis of new adsorbent

materials containing polysaccharides. Polysaccharides, stereoregular polymers of

monosaccharide (sugars), are unique raw materials in that they are: very abundant natural

polymers (they are referred to as biopolymers); inexpensive (low-cost) polymers; widely

available in many countries; renewable resources; stable and hydrophilic biopolymers; and

modifiable polymers. They also have biological and chemical properties such as non-toxicity,

biocompatibility, biodegradability, polyfunctionality, high chemical reactivity, chirality,

chelation and adsorption capacities. The excellent adsorption behavior of polysaccharides is

mainly attributed to: (1) high hydrophilicity of the polymer due to hydroxyl group of glucose

units: (2) presence of a large number of functional groups (acetamido, primary amino and/or

18

hydroxyl groups); (3) high chemical reactivity of these groups; (4) flexible structure of the

polymer chain73

. Figure 1.3 shows a molecular structure of cellulose.

Figure 1.3 Molecular structure of cellulose

The adsorption of gold (I,III) ion and its reduction into metallic form onto the adsorbents

derived from polysaccharide based biomass waste is considered to be promising technology

because feed materials used for adsorbents are waste themselves, easily available, and have

functional diversity; consequently, they have high potential as the adsorbents. In the present

research investigation, various bio-adsorbents were prepared for the recovery of gold from

aqueous solution. Among them, 6 different types of adsorbents were successfully prepared from

polysaccharides after treating with concentrated sulphuric acid by simple chemical modification

i.e. condensation reaction for cross-linking for the recovery of gold. For the preparation of

adsorbents, fixed amount of polysaccharide was treated with definite volume of the concentrated

sulphuric acid in order to cross-link by condensation reaction. After preparing the adsorbents, the

adsorption behaviors for gold as well as other precious and base metals (for the selectivity of

gold) from model and actual solution was systematically investigated in both batch and column

mode of operation.

19

1.5.1 Chemical Cross-linking

Chemical cross-linking of polysaccharide is a highly versatile method with good

mechanical stability. During cross-linking counter-ions diffused into the polymeric chain and

cross-linking agent reacts with polysaccharides forming either inter molecular or intra molecular

linkages. Factors which affect chemical cross-linking are agent concentration and reaction time.

The high concentration of cross-linking agent induces rapid reaction. Like physical cross-linking

high counter-ion concentration would require longer exposure times to achieve complete cross-

linking of the polysaccharides. In the present research work, cross- linking by condensation

reaction was carried out by the treatment with concentrated sulphuric acid onto the various

polysaccharides in order to increase the adsorption affinity of the adsorbents. The forms of the

natural and commercially available cellulose based polysaccharides are generally in crystalline

structure where adsorption of metal ions including gold is difficult so that it is cross-linked with

sulphuric acid. During such a cross-linking and acid treatment, it was cross-linked and converted

crystalline matrices of the polysaccharides into amorphous forms where gold ions were

effectively adsorbed which will be described in detailed in later chapters.

1.6 OBJECTIVES OF THE PRESENT RESEARCH WORK

Adsorptive separation and recovery of valuable metals especially gold from a wide

variety of sources such as primary sources like gold ore sample and from secondary sources like

electronic scraps and industrial wastes have been the subject of significant interest and also an

emerging issue74, 75

. Nowadays, recycling of electrical and electronic waste has attracted the

attentions of many researchers due to the presence of high content of valuable metals than

20

naturally occurring ores76

. The biomaterials, which are the natural poly-functional materials, are

environmentally benign, biodegradable and renewable; modification is possible in a number of

ways to enhance the adsorption selectivity and efficiency77

. Thus, being motivated by the serious

demand, continuously depleting ore and high values of gold, innovation of more economic and

environmentally benign methodologies is required for the separation, purification and recovery

of precious metals especially gold from various types of secondary resources too, the present

research investigation aims for the following points of overall or long-term objectives:

a. Development of low cost and environmentally benign bio-adsorbents from agricultural waste

biomass and commercially available various polysaccharides such as cotton, paper, cellulose,

dextran, pectic acid, and alginic acid by treating with concentrated sulphuric acid.

b. To investigate the adsorbent potentiality for the adsorptive separation, pre concentration and

recovery (in elemental form) of precious metals (especially gold) ions form acidic chloride

media in terms of selectivity, adsorption capacity and reusability of the cross-linked

polysaccharides.

c. To identify the suitable leaching and recovery method of gold and silver from primary source

such as Mongolian gold ore sample and secondary sources like scraps of plasma TV monitor

with low cost and effective recovery for the practical application.

d. To suggest the alternative method for the recovery of Au(I) from spent cyanide gold plating

solution to explore the possibility of using polysaccharide adsorbent, the specific or the

short term objectives of the present study are:

- To identify the new way of utilizing the polysaccharides of cotton and paper

21

- To prepare the low cost adsorbents from natural polysaccharides derived from cotton and

paper, biomass and commercially available polysaccharides for the effective separation and

recovery of targeted metal ion, i.e. gold

- To characterize the various polysaccharides adsorbents such as cotton, paper, cellulose,

dextran, alginic acid and pectic acid treated with sulphuric acid.

- To investigate and optimize the various adsorption parameters such as hydrochloric acid

concentration, solid to liquid ratio, temperature, shaking time etc for studying the adsorption

affinity, selectivity and maximum loading capacity of gold

- To demonstrate the evidence of Au(III) and Au(I) reduction into its elemental form, Au(0),

by optical microscopy and X- ray diffraction technique

- To investigate the more suitable, low cost and highly efficient recovery technique of gold

from the gold-loaded adsorbents.

- To explore the mechanism of gold adsorption followed by reduction by using investigated

adsorbents from hydrochloric acid media.

- To evaluate the recovery capacity and percentage purity of the gold recovered using

investigated polysaccharides based technology.

- To investigate the novel, environmentally benign and cost effective leaching process to strip

out the gold from primary and secondary sources.

- To provide the alternative of Au(I) recovery from sulphite media with the expectation that it

can be a promising method to recover gold from gold plating waste water.

22

1.7 OUTLINE OF THE THESIS

By the survey of the literature, it is known that there have been various studies regarding

the separation and recovery of precious metals. The environmental feasibility and economic

viability is the great concern for the selection of adsorbent for its selectivity, high adsorption

capacity and cost effectiveness towards the targeted metals ions. Based on the utilization of

waste biomass of polysaccharide this dissertation work was carried out in order to attain the

more scientific and practical approaches into the separation and recovery of precious metals from

aqueous as well as actual solution. To develop and optimize the operating conditions for this

research work, the thesis entitled “Development of the Low Cost Adsorbents from

Polysaccharides-based Biomass for the Recovery of Gold” is organized into the following

chapters. There are mainly two parts of the entire thesis. In the first Part (Chapters 2-5), the

adsorptive recovery of gold from model solution have been described and in the second part of

the thesis the dissolution of gold and silver from primary and secondary sources by acidic

thiourea solution and their subsequent recovery using cross-linked polysaccharide gel have been

investigated. The fundamental description of the precious metals and adsorption process for

recovering precious metals from various mixture solutions by different techniques together with

the biomass polysaccharide selection is thoroughly presented in Chapter 1.

The novel material for Au(III) recovery was developed from commercial cellulose by cross-

linking with concentrated sulphuric acid. The cross-linked cellulose showed very high affinity

towards gold and selectively adsorbed Au(III) from the mixture of other precious metals (Pt, Pd)

and base metals (Zn, Ni, Fe, Cu) in hydrochloric acid media. The work was also compared with

the other commercially available polysaccharides after cross-linking as in the case of cellulose in

Chapter 2. Although commercial cellulose after cross-linking yield very effective adsorbent for

23

Au(III), we have tried to extent our technology for the other low cost cellulose rich biomass thus

adsorbent was prepared from cotton by the similar method of cross-linking as in Chapter 2 and

investigate its adsorption behavior in Chapter 3. In Chapter 4 more abundantly distributed

waste paper was utilized for the recovery of gold together with the mixture solution of other

precious and base metals from acidic chloride media. Both the materials after cross-linking with

concentrated sulfuric acid selectively adsorbed gold with extremely high adsorption capacity.

Comparing with cotton, waste paper was much cheaper and abundant. From the results of

Chapters 2, 3 and 4, it was found that the cross-linked polysaccharide adsorbents were very

much effective for Au(III) recovery in hydrochloric acid media but the industrial gold plating

waste solution contains anionic complex of Au(I) sulphite or Au(I) cyanide. So that we further

tried to recover mono-valent gold using cross-linked cellulose gel from Au(I) sulfite solution in

sodium hypochlorite media in Chapter 5. For the application of the investigated polysaccharide

based adsorbent in actual practice, the real leached liquor of gold and silver was prepared from

both the primary source (Mongolian gold ore sample) and the secondary source (scraps of

plasma TV monitor) and successfully recovered gold and silver by leaching with acidic-thiourea

followed by its adsorptive recovery using cross-linked cotton gel is presented in Chapters 6 and

7, respectively. Finally, the overall concluding remarks and an outlook are suggested in Chapter

8. At last part of this thesis, a list of publications related to author’s works, list of presentation

and contributions to scientific forum are summarized in appendices.

24

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http://ewasteguide.info/files/Hageluecken_2009_R09.pdf

74. Fujiwara K, Ramesh A, Maki T, Hasegawa H, Ueda K, (2007) Adsorption of platinum(IV),

palladium(II) and gold(III) from aqueous solutions onto l-lysine modified cross-linked

chitosan resin, J. Hazard. Mater., 146, 39-50

75. Park YJ, Fray D, (2009) Recovery of high purity precious metals from printed circuit boards,

J. Hazard. Mater., 164, 1152-1158.

32

CHAPTER CHAPTER CHAPTER CHAPTER 2222

Chemical Modification of Some Commercially Available Polysaccharides for

Selective Recovery of Gold from Acidic Chloride Media

The research work was conducted for developing new adsorbents which may be successful to

adsorb and subsequently reduced the Au(III) to its elemental form without adding any types of reducing

agents. For this purpose, commercially available cellulose powder was first cross-linked with

concentrated sulphuric acid in order to create the new coordinating sites for Au(III) adsorption. During

the treatment, crystalline form of commercial cellulose which has negligible affinity for Au(III) ion, was

converted into amorphous form with very high affinity for Au(III) ion. The trivalent gold was selectively

adsorbed onto the cross-linked cellulose gel from the mixture of other precious and base metals. It was

visually observed that yellow shining metallic particle was formed and floated at the surface of the reactor

at the short contact time whereas it was found that heavy particle was aggregated then formed after long

time contact which was settled down on the bottom later. It was further confirmed from the observation of

crystalline peaks of elemental gold [reduced gold Au(0) in XRD profile of Au(III) loaded cross-linked

cellulose gel. So, based on the results, we further expect to attempt whether such quantitative and

selective gold adsorption is common to all kinds of polysaccharides or not, we employed commercially

available polysaccharide such as dextran, alginic acid and pectic acid in which the hydroxyl groups as a

major functional one can be cross-linked with concentrated sulphuric acid similar to the case of cellulose

gel. In all cases that the high selectivity for Au(III) ion from the mixed solution was found, thus the

polysaccharide-based adsorbents investigated in this study can be expected as promising materials to be

employed in large scale industrial processing for gold recovery.

33

2.1 INTRODUCTION

The increasing amount of electronic and electrical wastes is creating harmful impacts on

environment and also causes serious problems with regard to the supply of raw materials.

Generally, electronic waste encompasses a broad and growing range of electronic devices such

as refrigerators, air conditioners, hand-held cellular phones, personal stereos, and computers1. In

most cases the wastes generated from these products contain appreciable amounts of valuable

metals. For example, the content of gold in these wastes discarded in this form is much higher

than that in its ore form2.

Recycling and reuse of metals, in particular, is very crucial for

preventing the practice of excessive mining and supply of primary metals. Nowadays, precious

metals are extensively used in many fields such as catalyst of various chemical processes,

electrical and electronic industries, medicine and jewelry3. Taking into consideration of retarding

resource of precious metals like gold with its ever increasing applications, it is important to

develop the process for its recovery and recycle from spent appliances4. Consequently, the most

suitable recovering and reuse technologies can contribute to reduce the dependencies on primary

metal resources5. In the last decade, biopolymer-based products had been increasingly studied

with the focus on polysaccharides. Mainly cellulose, the most important renewable resource, was

studied as a starting material for the chemical modification. Cellulose and its derivatives attract

significantly the attention of researchers due to their outstanding physical and chemical behavior6

as well as of their abundances in nature, renewability, low cost and availability.

Native cellulose exhibits only poor adsorption for majority of metal ions because of the

crystalline structure and absence of particular functional groups with high affinity for metal ions.

However, proper modifications by chemical reactions can enhance adsorption capacity and

structural stability of native cellulose. In our previous study, we developed dimethylamine-

34

immobilized (DMA) paper gel for recovering precious metals7 which exhibited the maximum

adsorption capacity for Au(III) as high as 4.6 mol kg-1

. However, it required a lot of chemical

reagents and high cost to prepare DMA-paper gel from spent paper, one of cellulosic materials,

though the feed material is cheap. In the present work, we prepared adsorbents from typical

polysaccharides like cellulose, dextran, alginic acid and pectic acid by treating with concentrated

sulphuric acid to observe their adsorption behavior for metal ions, especially for Au(III). That is,

we investigated the adsorption of Au(III) on cellulose gel in details and qualitatively compared

with those of other polysaccharide gels.

2.2 EXPERIMENTAL PROCEDURE

2.2.1 Materials

Analytical grade chloride salts of copper (Katayama Chemical, Japan), zinc (Sigma

Aldrich), iron and palladium (Wako, Japan) were used to prepare test solutions of respective

metals. Analytical grade HAuCl4•4H2O and H2PtCl6•6H2O (Wako, Japan) were used to prepare

gold and platinum solutions, respectively. All other reagents were of analytical grade and were

used without further purification.

2.2.2 Preparation of Cross-linked Polysaccharide Adsorbents

Crystalline cellulose powder (102330 Cellulose) marketed by Merck, Germany, for thin

layer chromatography was used as the feed material of cellulose gel. Dextran, produced by

Leuconostoc mesenteroides, strain No. B-512, average molecular weight of which is around

200,000 (Sigma Aldrich), was employed as the feed material for dextran gel. Sodium alginate,

500 cps (Sigma Aldrich) and polygalacturonic acid (Nacalai Tesque, Inc., Kyoto, Japan) were

35

used for the feed materials for alginic acid and pectic acid gels, respectively. For the preparation

of cross-linked polysaccharides gels, for example, 10 g of cellulose (MW 200,000) powder was

mixed with 50 cm3 of concentrated sulphuric acid and stirred for 24 h at 373 K using reflux

condenser for cross-linking condensation reaction. Then, after the mixture was cooled, it was

neutralized with NaHCO3, and washed several times with distilled water until neutral pH. The

black product obtained was dried in a convection oven for 24 h at 343 K. Then, dried mass of the

product was ground to regulate the uniform particle size of 75-100 µm. Thus prepared gels were

termed as cross-linked cellulose (CLPC). The gels of dextran, alginic acid and pectic acid were

also prepared by the same method; these were termed as cross-linked dextran (CLD), cross-

linked alginic acid (CLAA) and cross-linked pectic acid (CLPA), respectively.

2.2.3 Measurement and Analysis

The metal concentrations in the adsorption tests were measured by using Shimadzu

model ICPS-8100 ICP/AES spectrometer and Shimadzu model AAS-6800 atomic absorption

spectrophotometer. The surface morphology of the adsorbents before and after gold recovery

process was recorded using a JEOL (Tokyo, Japan) model JCM-5100 scanning electron

microscope (SEM) at acceleration energy of 20 kV. Visual observation was recorded using

KEYENCE model VHX/VH series optical microscope. The generation of elemental gold (Au0)

on the gels after the adsorption of Au(III) was elucidated by means of X-ray diffraction by using

Shimadzu model, XRD-7000, X-ray diffractometer. Spectroscopic studies were performed by

JASCO model FT/IR-410 Fourier transform infrared spectrometer.

36

2.2.4 Batch-wise Mode of Adsorption Tests

In the present investigation, the adsorption behavior of gold and other metal ions was

tested individually in batch-wise experiment. In a typical experimental run, each 10 mg of the

dried CLPC, CLD, CLAA and CLPA gels were shaken together with 10 cm3 of test solution

containing 0.1 mM (M = mol dm-3

) of each metal ion in varying concentration of hydrochloric

acid at 303 K for 48 h. After equilibrium, the mixture was filtered and the filtrate was analyzed

to determine the remaining metal ion concentrations.

For adsorption isotherm studies, a series of Au(III) solution were prepared in 0.1 mol dm-

3 hydrochloric acid in which the initial concentration of Au(III) was varied in the range of 0.5-16

mmol dm-3

. Ten milligrams of the dried gel was added into the 10 cm3 of Au(III) solutions to be

shaken at four different temperatures (293, 303, 313 and 323 K) for CLPC gel and at only 303 K

by CLD, CLAA and CLPA gels for 150 h to attain complete equilibrium. Percentage adsorption

for each metal ion was calculated according to Eq.(2.1), where Ci and Ce (mmol dm-3

) are the

initial and equilibrium concentrations of metal ion, respectively. The adsorption capacity was

calculated by the mass balance of Au(III) before and after the adsorption according to Eq.(2.2).

% Adsorption = �� − ��

��× 100 … … … … … … … … . �2.1�

And,

Q = �� − ��

� × � … … … … … … … … … … … … . . �2.2�

where Q (mmol g-1

) represents the adsorption capacity, V (dm3) is the volume of the test solution

used and W (g) is the dry weight of the adsorbent.

The adsorption kinetics of Au(III) were measured in detail for CLPC gel as follows. Two

hundred milligrams of the dried CLPC gel was shaken together with 200 cm3 of 2 mmol dm

-3 of

37

Au(III) solution in 0.1 mol dm-3

hydrochloric acid at four different temperatures (293, 303, 313

and 323 K). Suspended mixture (3.5 cm3) was immediately sampled at definite time intervals

from the start of the operation to measure the time variation of the residual Au(III) concentration

after filtration.

2.2.5 Measurement of the Degree of Crystallinity of Cellulose

For the sample of cellulose employed in the present work, the crystallinity index before

and after cross-linking was measured by means of X-ray diffraction technique. The peak height

method was employed to calculate the Crystallinity Index (CrI). A wide range of the reported

values for native cellulose by X-ray diffraction, in the range 62.0-87.6% using peak height

method and from 39.0 to 75.3% using other methods were found in the many literature8. X-ray

diffraction patterns of cellulose samples were measured by using Shimadzu model. X-ray

diffractometer (XRD-7000), at room temperature from 5 to 40º using Cu/Kα1 irradiation (1.54 Ǻ)

at 40 kV and 30 mA. The scanning speed was 2º min-1

and the data were collected in continuous

mode. According to the method as proposed by Segal et al., 1959, the peak intensity method was

used according to Eq.(2.3)9.

CrI = �� − ��

��× 100 … … … … … … … … … … . … … . �2.3�

where, I002 is the intensity of the peak at 2θ = 22.5º and Iam is the minimum intensity

corresponding to the amorphous content at 2θ = 18º.

2.3 RESULTS AND DISCUSSION

2.3.1 Effect of Hydrochloric Acid Concentration for the Adsorption of Metal Ions

38

Figure 2.1 shows the effect of hydrochloric acid concentration on the adsorption of

Au(III), Pt(IV), Pd(II), Cu(II) and Zn(II) ions on CLPC from their individual solutions. No metal

ions were adsorbed at all except for Au(III), which was nearly quantitatively adsorbed over the

whole concentration region of hydrochloric acid tested, suggesting that CLPC gel exhibited very

high affinity for Au(III) over other precious and base metal ions tested. Similar phenomena were

also observed for CLD, CLAA and CLPA. From these results, it may be expected that low-cost

adsorption gels with excellent selectivity to Au(III) can be prepared from a variety of

polysaccharides by a simple treatment using concentrated sulphuric acid.

Figure 2.1 Effect of hydrochloric acid concentration for the adsorption of various metal ions

onto CLPC adsorbent. Conditions: volume of solution = 10 cm3, concentration of metals = 0.1

mmol dm-3

, dry weight of gel = 10 mg, shaking time = 48 h, temperature = 303 K

2.3.2 Surface Analysis of the Adsorbents

The surface morphologies of the adsorbents (cellulose and dextran) after treatment with

concentrated sulphuric acid are shown in Figure 2.2. The Figures 2.2 (a, c) showed that the dry

0

20

40

60

80

100

0 1 2 3 4 5 6

Pe

rce

nta

ge

, %

[HCl]/ mol dm-3

Au (III)

Pt (IV)

Pd (II)

Fe (III)

Cu (II)

39

gel surfaces before gold adsorption were smooth and lacked of cracks and holes. The SEM

images of cellulose and dextran gels after gold recovery are also shown in the same Figures 2.2

(b, d). The elemental gold particles were clearly seen with some rough surfaces compared to

Figures 2.2 (a, c). Because both the gels are non-porous, significant physical adsorption is not

likely to occur. In practical point of view, such types of surface structures are considered to be

beneficial for the recovery of gold in its elemental form.

Figure 2.2 SEM micrographs of (a) cross-linked cellulose, (b) cellulose after gold adsorption,

(c) cross-linked dextran, and (d) dextran after gold adsorption taken at 20 kV acceleration energy,

at 1500x magnification, scale = 5 µm, respectively

(c (d

5 µm 5 µm

(b(a

5 µm 5 µm

40

2.3.3 Adsorption Isotherm Studies

Since all types of the modified polysaccharide gels prepared in the present work were

observed to be distinctively selective for gold, subsequent experiments were carried out only for

Au(III) ion using the polysaccharide gels mentioned earlier. Figure 2.3 (a) shows the adsorption

isotherms of Au(III) on CLPC, CLD, CLAA and CLPA gels at 303 K for comparison. In all

cases, the amount of adsorption increased with increasing concentration of gold in the low

concentration region whereas it tended to approach constant values corresponding to each gel in

the high concentration region, suggesting the monolayer adsorption according to the Langmuir

isotherm model. Consequently, the adsorption isotherm data were analyzed on the basis of the

Langmuir adsorption model expressed by Eq.(2.4), in which it is assumed that the adsorption

took place on a homogenous monolayer surface with identical adsorption sites, and there is no

lateral interaction between the adsorbed sites.

��

= 1

�� • � + ��

… … … … … … … … … … … … … … . �2.4�

where, Qe(mmol g-1

) represents the equilibrium adsorption capacity, Qmax(mmol g-1

) represents

the maximum adsorption capacity, b (dm3 mmol

-1) is the Langmuir constant related to the

adsorption energy.

Ce/mmol dm-3

q/m

mo

lg

-1

41

Figure 2.3 Adsorption isotherms of Au(III) on cross-linked cellulose, cross-linked dextran,

cross-linked alginic acid, and cross-linked pectic acid, adsorbents (a) Experimental plots and (b)

Langmuir plots. Conditions: volume of the feed solution = 10 cm3, weight the dried gel added =

10 mg, shaking time = 150 h, [HCl] = 0.1 mol dm-3

and temperature = 303 K

The plot of Ce q-1

versus �� is presented in Figure 2.3 (b). As expected from the

Langmuir model, linear relationship with high correlation coefficient values are observed for

each type of gels studied. The values of maximum adsorption capacities (qmax) and Langmuir

constant (b) for Au(III) for each gel were calculated from the slope and intercept of these straight

lines as shown in Table 2.1. The maximum adsorption capacities at 303 K are in the following

orders, CLPC>CLD>CLAA>CLPA.

Ce/mmol dm-3

Ceq

-1/g

dm

-3

42

Table 2.1

Langmuir isotherm parameters for Au(III) onto cross-linked polysaccharide gels

Adsorbents qmax (mmol g-1

) b (dm3 mmol

-1) R

2

Cross-linked cellulose 7.57 7.03 0.994

Cross-linked dextran 7.20 10.36 0.996

Cross-linked alginic acid 5.64 27.86 0.998

Cross -linked pectic acid 4.80 52.05 0.999

The maximum adsorption capacity for Au(III) at 303 K were compared to various

adsorbents reported in literatures as listed in Table 2.2. Among all the reported adsorbents, the

maximum adsorption capacity of CLPC gel was found to be the highest value, which suggests

the possibility of recovering Au(III) from industrial effluent using this adsorbent. Such high

adsorption capacity of the cross-linked cellulose may be attributable to the change in the

polymeric structure and decrease in crystallinity index after acid treatment, which will be

discussed in later sections. Since the highest adsorption of Au(III) was observed for CLPC gel,

detailed works were conducted only for CLPC gel in the subsequent work.

43

Table 2.2

Comparison of maximum adsorption capacities for Au(III) on different adsorbents

Adsorbents Maximum uptake capacity Adsorption media References

(mmol g-1

)

Cross-linked cellulose gel 7.57 0.1 M HCl Present study

Cross-linked dextran gel 7.20 0.1 M HCl Present study

Cross-linked alginic acid gel 5.64 0.1 M HCl Present study

Cross-linked pectic acid gel 4.80 0.1 M HCl Present study

Glycine modified cros-linked chitosan 0.86 pH 2 [3]

Dimethyamine-paper 4.6 1 M HCl [7]

р-Aminobenzoic acid -paper 5.1 1 M HCl [7]

Persimmon waste 4.95 0.1 M HCl [10]

Cross-linked lignophenol 1.92 0.5 M HCl [11]

Primary amine-cross-linked lignophenol 1.98 0.5 M HCl [12]

Ethylene diamine cross-linked lignophenol 3.08 0.5 M HCl [12]

Dimethyamine cross-linked lignophenol 7.2 0.5 M HCl [13]

Chemically modified chitosan with magnetic3.43 pH 0.5 [14]

Cross-linked microalgae 3.25 0.1 M HCl [15]

2.3.4 Effect of Temperature for the Adsorption of Au(III) onto CLPC Adsorbent

Figure 2.4(a) shows the adsorption isotherms of Au(III) on CLPC gel at four different

temperatures, that is 293, 303, 313 and 323 K. As seen from Figure 2.4 (a), temperature greatly

affected the adsorption of Au(III) on CLPC gel; i.e. the increase of temperature enhanced the

adsorption of Au(III) on CLPC gel. Similar to the preceding section, the results were re-plotted

on the basis of the Langmuir adsorption model as shown in Figure 2.4 (b). The maximum

44

adsorption capacity and Langmuir constant were evaluated from the slopes and intercepts of the

straight lines in this figure at each temperature studied as listed in Table 2.3. It was obvious that

the adsorbed amount of gold increased with increasing temperature which suggests that the

adsorption of Au(III) on CLPC gel was an endothermic process.

Table 2.3

Thermodynamic parameters for Au (III) by cross-inked cellulose Adsorbent

Temperature b qmax �Gº �Hº �Sº R2

(K) (dm3 mmol

-1) (mmol g

-1) ( kJ mol

-1) (kJ mol

-1) (J K

-1 mol

-1)

293 1.658 3.891 -1.231 0.936

303 7.333 7.575 -5.020 102.170 353.261 0.994

313 26 9.615 -8.480 0.999

323 82 12.195 -11.836 0.999

From the temperature dependency of the Langmuir constant, or adsorption equilibrium

constant (b), the thermodynamic parameters such as changes in the free energy (∆G°), enthalpy

(∆H°) and entropy (∆S°) associated to the adsorption process of Au(III) on CLPC gel were

evaluated according to Eqs. (2.5) and (2.6).

∆G° = - RT lnb……………………………….………………(2.5)

lnb = - ∆G°/RT = - ∆H°/RT + ∆S°/R ……………………….(2.6)

The plot lnb as a function of 1/T yields a straight line as depicted in Figure 2.4 (c) for

CLPC gel, from which values of ∆H° and ∆S° were calculated from the slope and intercept,

respectively. The negative values of Gibbs free energy (∆G°) at all temperatures studied confirm

the feasibility of the process and spontaneous nature of adsorption. Positive value of enthalpy

(∆H°) demonstrated the endothermic nature of the adsorption. Entropy, as the measure of

45

freedom of the system, also indicated the positive values, suggesting the increased randomness at

solid-solution interface during the adsorption of Au(III) on CLPC gel.

Ce/mmol dm-3

q/m

mo

lg

-1

Ce/mmol dm-3

Ceq

-1/g

dm

-3

46

Figure 2.4 Effect of temperature for the adsorption of Au(III) onto CLPC adsorbent (a)



-3

2.3.5 Adsorption Kinetics of Au (III) onto CLPC Adsorbent

Figure 2.5 (a) shows the time variation of the adsorbed Au(III) amount on CLC

adsorbent at various temperature. The rate of adsorption rapidly increased at the initial stage of

adsorption and tended to approach a constant value or equilibrium value after certain period of

shaking. It was evident from this results that although it took for very long time to attain

equilibrium at low temperature like 293 and 303 K, such slow kinetics were improved by

increasing temperature of the system.

The rate of adsorption of the Au(III) onto the CLPC gel at the initial stage was

analyzed on the basis of pseudo-first order kinetic model expressed by Eq.(2.7) as shown in

47

Figure 2.5 (b), in which all the plots cluster on the straight lines passing through the origin

corresponding to different temperatures.

lnCt/Ci = - kt………………………………………………(2.7)

where Ct and Ci stand for the concentration of Au(III) at time t, and initial concentration,

respectively; k is the pseudo-first order kinetic rate constant (h-1

) and t is the time (h).

From the slopes of these straight lines, the pseudo-first order rate constants were evaluated

for four different temperatures. From the rate constants k evaluated at different temperatures, the

activation energy of the adsorption of Au(III) on CLPC gel was evaluated according to the

Arrhenius equation expressed by Eq.(2.8).

ln k = ln A – Ea/R (1/T)……………………………………………..(2.8)

where R is the universal gas constant, Ea is the activation energy, A is the Arrhenius constant and

T is the absolute temperature in Kelvin. Figure 2.5 (c) shows the plot of lnk versus 1/T according

to Eq.(2.8). From the slope of this straight line, the value of the activation energy (Ea) was

evaluated as 71.8 kJ mol-1

. This value suggests that the adsorption of Au(III) onto CLPC gel is a

chemical adsorption, which is in accordance with that the magnitude of activation energy for

chemical adsorption is usually between 8.4 to 83.7 kJ mol-1

. The positive value of Ea suggested

that an increase in temperature favors the adsorption.

48

Continued

(b)

t/h

lnC

e/C

i

(a)

q/m

mo

lg

-1

t/h

49

Figure 2.5 Adsorption kinetics of Au(III) onto CLPC adsorbent at different temperature (a)

experimental plot, (b) pseudo-first order plot, and (c) Arrhenius plot. Conditions: weight of the

dry gel added = 200 mg, volume of the feed solution = 200 cm3, [Au(III)] = 2 mmol dm

-3, [HCl]

= 0.1 mol dm-3

2.3.6 Crystalline Structure of Cellulose

Figure 2.6 shows the X-ray diffraction patterns of raw cellulose and CLPC gel. The value

of CrI of the former was evaluated as 83% while it was found to be decreased to 31% after the

treatment by concentrated sulphuric acid. The higher the crystalline structure, the less is the

chemical activity in the cellulose. Because majority of reagents in aqueous solution can only

penetrate into the amorphous parts of cellulose, these domains are also termed the accessible

regions of cellulose, and crystallinity and accessibility are closely related16

. A highly crystalline

cellulose sample has a tight structure with cellulose chains closely bound to each other, leaving

too little space for chemical reaction anywhere within the cellulose crystal. It was obvious from

Figure 2.6 that the treatment by concentrated sulphuric acid disrupted the crystalline structure of

(c)

50

cellulose and turned to amorphous structure which lead to the very high and selective adsorption

of Au(III).

Figure 2.6 X-ray diffraction pattern of raw cellulose (a, multiple peaks) and amorphous cellulose

(b, single smooth peak) generated with concentrated sulphuric acid (96%) treatment. X-axis:

Bragg angle (2θ). I002 represents the maximum intensity at 2θ = 22.5º. Iam shows the minimum

intensity at 2θ = 18º used to calculate crystallinity in the peak height method

2.3.7 Analysis of Cross-linked Polysaccharide Adsorbents After Au (III) Adsorption

The instrumental analysis of the gels after the adsorption of Au(III) were carried out in

order to ascertain the formation of elemental gold during adsorption for all types of gels tested.

That is, the X-ray diffraction analyses and visual observations of CLPC, CLD, CLAA and CLPA

gels were performed after the adsorption of Au(III). Almost same patterns of peaks and gold

aggregations were observed for all kinds of adsorbents. XRD spectrum observed for CLPC, CLD,

0

500

1000

1500

2000

2500

5 15 25 35

Inte

nsi

ty

2θ [degree]

(a)

(b)

51

CLAA and CLPA gels are shown in Figure 2.7. The sharp peaks observed at 2θ values of

around 38, 44, 64 and 77 degree in this figure were identified the crystal structure of elemental

gold, verifying the generation of Au(0) on the polysaccharide gels after the adsorption, which

suggested that Au(III) underwent subsequent reduction to Au(0) on the surface of

polysaccharides after treatment by concentrated sulphuric acid.

The formation of elemental gold was further reconfirmed from the optical microscope

photographs as shown in Figure 2.8 for these gels in which very fine and clear aggregated

particles of elemental gold with various shapes are distinctly visible after long period of shaking

at high concentration of Au(III) solution. In another hand, for short shaking period at low

concentration of Au(III) for example, 0.2 mmol dm-3

, only microscopic gold particles were

observed to be float on the surface of sample solution. Both results of XRD and observation

analyses strictly confirmed as a reliable proofs for the reductive adsorption of Au(III) on

different gels prepared from simple chemical modification using various polysaccharides.

Extremely high selectivity and loading capacity of these gels for Au(III) may be attributed to the

reduction of Au(III) to Au(0) during the adsorption. From the results of Figure 2.8, it can be

inferred that gold particles formed by the reduction process have a certain sort of tendency to be

detached from the gel surface forming clear aggregates. Similar observation had been also

reported in our previous study in the adsorption on chemically modified spent paper gel7.

52

Figure 2.7 XRD patterns of gold loaded cross-linked cellulose, cross-linked dextran, cross-

linked alginic acid and cross-linked pectic acid, adsorbents

Inte

nsit

y

10 20 30 40 50 60 70 80

2θ [Degree]

CLC

CLD

CLAA

CLPA

53

Figure 2.8 Optical microscope photographs of gold-loaded cross-linked (a) cellulose (b) dextran

(c) alginic acid and (d) pectic acid, adsorbents

2.3.8 Measurement of Fourier Transformed Infrared Spectra

The structural analysis of the polysaccharide gels before and after cross-linking, and after

the adsorption of Au(III) on each type of gels were carried out by means of the observation of IR

spectra in order to infer the binding mechanism of Au(III) ions onto the surface of the gels.

Figures 2.9 (A) and (B) show the infrared spectra for cellulose and dextran, respectively. In the

spectra for crude materials (a), in Figures 2.9 (A) and (B) and also in alginic acid and pectic acid

gels (figures not included), there observed a broad band centered at around 3450 cm-1

due to O-H

stretching, a sharp band at round 3050 cm-1

is considered to be attributable to the C-H stretching,

(b)

(d) (c)

(a)

54

another sharp band at 1720 cm-1

due to C=O stretching and a broad band centered at 1100 cm-1

due to C-O stretching and O-H bending of alcoholic groups.

The peaks observed at around 1780 and 1690 cm-1

after acid treatment in Figures 2.9 (A)

and (B) and also in the alginic acid and pectic acid (figures not included) are ascribed to the

carbonyl or carboxylate stretching frequencies. Thus, it was inferred that, during the acid

treatment, long polymeric chains of polysaccharides were cleaved to shorter chains which were

then subjected to condensation cross-linking reaction with the treatment of concentrated

sulphuric acid that was confirmed by the appearance of C-O-C stretching at around 1200 cm-1

observed in all kinds of polysaccharide gels.

Figure 2.9 FT-IR spectra of (a) crude, (b) cross-linked, and (c) gold-loaded adsorbents of (A)

cellulose, and (B) dextran

B A

(a)

(b)

(c)

Wave number 40011001800250032003900

% T

ran

smit

tan

ce

Wave number (cm-1)

(b)

(c)

(a)

55

After Au(III) adsorption as shown in spectra (c) of Figures 2.9 (A) and (B) and for gold

-loaded gels of pectic acid and alginic acid (figures not included), the absorption bands due to

hydroxyl groups have flattened and shifted to lower frequency region (centered at around 3410

cm-1

). Since coordination with metal ion weakened the O-H bond resulting in absorption at a

lower frequency, shifting of O-H stretching band to lower frequency region was attributed to the

coordination of hydroxyl groups with loaded Au(III) ion. In addition, intensity of the absorption

band attributed to C=O stretching of carbonyl groups at around 1690 cm-1

has increased after

Au(III) loading as shown in spectra (c) of Figures 2.9 (A) and (B) and also for other gels

studied.

2.3.9 Proposed Adsorption-Reduction Mechanism

The mechanism through which particular adsorbent binds the targeted metal ions has

recently received increased attention as the advantages of understanding such mechanism

become more apparent.

Based on the spectroscopic analysis mentioned in the preceding section, the mechanism

of adsorption of Au(III) was inferred as follows. The cleaved chain of polysaccharide in presence

of concentrated sulphuric acid undergoes the condensation cross-linking reaction as shown in

Scheme 2.1 in the case of cellulose, for example, with the release of water molecules. Thus, the

prepared gel was inferred to contain new ether bond linkages of C-O-C as observed in IR

spectrum mentioned earlier. The oxygen atoms of these new C-O-C bonds play important roles

as the coordination sites for Au(III) together with oxygen atoms of un-reacted hydroxyl groups

and those of C-O-C bonds existed before the condensation reaction to form stable 5-membered

chelate rings. In acidic chloride media, majority of Au(III) exists in the form of tetrachloro-

56

complex, AuCl4-. This auric chloride complex was stable in acidic and oxidizing media. When

contacted with the cross-linked polysaccharide gels, Au(III)-chloro complex was coordinated

with the above-mentioned many oxygen atoms of the acid treated polysaccharides and oxygen

atoms of ether bond. Further, during this adsorption process, some hydroxyl groups were

oxidized to carbonyl groups. Such hydroxyl group play an important role for the reduction of

Au(III) to Au(0). Besides this, as we carried out the experimental works in acidic chloride media,

some of the chloride ions were also responsible for the donation of electron for reducing the

Au(III) to Au(0).

It is well known that Au(III) is easy to be reduced because it has high redox potential (1.0

V) in comparison to other metal ions like Pd2+

(0.68 V) and Pt4+

(0.62 V). Further, the change in

the polysaccharide structures after acid treatment played the characteristic role in the

improvement of the stability of the gels. Thus, it was inferred that these were the driving force

for the formation of elemental gold particles, resulting in the extraordinary high selectivity and

loading capacity. Also in the case of other polysaccharide gels treated with concentrated

sulphuric acid like CLD, CLAA and CLPA, Au(III) ions are presumed to be adsorbed onto the

gel surface and further reduced to metallic gold particles in the similar mechanism as shown

below.

From the results of FTIR spectra, hydroxyl groups of polysaccharide gels were oxidized

to carbonyl group, which is expressed as Eq. (2.9),

……………………………….……(2.9)

The net reaction takes place as Eq. (2.10),

Au (III) + 3e- Au

0(s) ……………………….………………….(2.10)

4

32

1

O5

HOH2C

H

HO

+ 2H+ +2e-4

3

2

1

O5

HOH2C

O

57

Based on the Eqs. (2.9) and (2.10), the overall reaction is expressed as Eq. (2.11)

Au(III)(aq) + Au(0)

(s) + +4H+

+ e-…………. (2.11)

O

H

H

H

OH

O

H

H

H

OO

O Au

O

O

H

Cl

Cl

Cl

H

O

n

m

O

H

H

H

O

H

H

H

OO

O

O

H

H

O

n

m

O

O

CH2OH CH2OH

CH2OH CH2OH

H

O

H

H

H

H

O

H

H

H

H

OO

O

O

H

H

O

n

m

CH2OH

CH2OH

OH

OH

O

H

H

H

H O

HO

n

CH2OH

OHOH

O

H

H

HH

OO

Hm

CH2OH

OHOH

+

Cellulose

Conc.H2SO4, 1000C

H2O

AuC

l 4- ads

orpt

ion

Au(III) reduction+ Au0 +4H+ + 3Cl-

Cross linked cellulose

Au(III) adsorbed cellulose gel Formation of elemental gold on cellulose gel

+ Cl-

H

H

Scheme 2.1 Inferred example of synthetic route of cross-linked cellulose and its adsorption-

reduction mechanism

4

3

2

1

O

5

HOH2C

H

HO

4

32

1

O5

HOH2C

O

2 2

58

2.4 CONCLUSIONS

The results obtained in the present work suggested that different kinds of polysaccharides

can be used as effective adsorbents for Au(III) by simple treatment with concentrated sulphuric

acid due to their high affinity and selectivity towards Au(III) in acidic chloride media at ambient

temperature. The Langmuir adsorption isotherm was found to be best fit to the experimental data

with the maximum adsorption capacities at 303 K to be of 7.57, 7.20, 5.64 and 4.80 mmol g-1

for

cellulose, dextran, alginic acid and pectic acid gels respectively. Analysis of kinetic data suggests

that the uptake of Au(III) on cellulose gel followed the pseudo-first order kinetics. The activation

energy evaluated from Arrhenius equation was found to be 71.8 kJ mol-1

for cellulose gel.

Thermodynamic studies showed the spontaneously favorable, endothermic and increased

disorderness process during the adsorption of Au(III) by cross-linked cellulose gel. The results

obtained from the solid state analysis (XRD spectrum and optical microscope photographs) for

all kinds of gels after the adsorption of Au(III) revealed the noticeable observation of elemental

gold particles from ionic form i.e. Au(III). Based on the observation of infrared spectra the

mechanism of Au(III) adsorption was discussed, and it was inferred to involve an apparent redox

reaction; the ionic gold species was reduced to the metallic form using chloroauric complex of

gold.

Thus, present study demonstrated that gold recovery was successfully achieved using

various polysaccharides after simple treatment with concentrated sulphuric acid as an alternative

to traditional adsorbents employed in industrial gold recovery.

59

REFERENCES CITED

1. Li J, Miller JD, (2007), Reaction kinetics of gold dissolution in acid thiourea solution using

ferric sulfate as oxidant, Hydrometallurgy 89, 279-28.

2. Huang X, Wang Y, Liao X, Shi B, (2010) Adsorptive recovery of Au3+

from aqueous

solutions using bayberry tannin-immobilized mesoporous silica, J. Hazard. Mater., 183, 793-

798.

3. Ramesh A, Hasegawa H, Sugimoto W, Maki T, Ueda K, (2008) Adsorption of gold(III),

platinum(IV) and palladium(II) onto glycine modified cross-linked chitosan resin, Biores.

Technol., 99, 3801-3809

4. Parajuli D, Kawakita H, Inoue K, Ohto K, Kajiyama K (2007) Persimmon peel gel for the

selective recovery of gold, Hydrometallurgy 87, 133-139.

5. Manhart A (2010) International cooperation for metal recycling from waste electrical and

electronic equipments, J Ind Ecol., 15, 13-30.

6. Sena C, Godinho MH, Oliveira CLP, Figueiredo Neto A M, (2011) Liquid crystalline

cellulosic elastomers: free standing anisotropic films under stretching, Cellulose, 18, 1151.

7. Adhikari CR, Parajuli D, Kawakita H, Inoue K, Ohto K, Harada H, (2008) Dimethylamine

modified waste paper for the recovery of precious metals, Environ. Sci. Technol. 42, 5486-

5491.

8. Gama FM, Teixeira J A, Mota M, (1994) Cellulose morphology and enzymatic reactivity- a

modified solute exclusion technique, Biotechnol. Bioeng., 43, 381.

9. Segal L, Creely JJ, Martin AE, Conrad CM, (1959) An empirical method for estimating the

degree of crystallinity of native cellulose using the X-ray diffractometer, Text Res. J., 29,

786.

60


of precious metals by persimmon waste chemically modified with dimethylamine, J. Hazard.

Mater., 100, 4083-4089.

11. Parajuli D, Adhikari CR, Kuriyama M, Kawakita H, Ohto K, Inoue K, Funaoka M, (2006a)

Selective recovery of gold by novel lignin-based adsorption gels. Ind. Eng. Chem. Res., 45,

8-14.

12. Parajuli D, Kawakita H, Inoue K, Funaoka M, (2006b) Recovery of gold(III), Palladium(II),

and Platinum(IV) by aminated lignin derivatives. Ind. Eng. Chem. Res, 45, 6405-6412.

13. Parajuli D, Khunathai K, Adhikari CR, Inoue K, Ohto K, Kawakita H, Funaoka M, Hirota K,

(2009), Total recovery of gold platinum and palladium using lignophenol derivatives, Miner.

Eng., 22, 1173.

14. Donia AM, Atia AA, Elwakeel KZ, (2007) Recovery of gold(III) and silver(I) on a

chemically modified chitosan with magnetic properties, Hydrometallurgy, 87, 197.

15. Khunathai K, Xiong Y, Biswas BK, Adhikari BB, Kawakita H, Ohto K, Inoue K, Kato H,

Kurata M, Atsumi K, (2011) Selective recovery of gold by simultaneous adsorption–

reduction using microalgal residues generated from biofuel conversion processes, J. Chem.

Technol. Biotechnol., 87, 393.

16. Li Q, Chai L, Yang Z, Wang Q, (2009) Kinetics and thermodynamics of Pb(II) adsorption

into modified spent grain from aqueous solutions, Appl. Surf. Sci., 255, 4298-4303.

61


Development of Bio-adsorbent for Selective Recovery of Au(III) from Water

Using Cotton Cellulose Treated with Concentrated Sulphuric Acid

Gold is widely used precious metal to make traditional jewelry as well as in different electronic

devices including computer, mobile phone and other high tech electronic appliances. Due to technological

development, old electronic appliances were replaced by more sophisticated and newly developed

electronic devices so that large amount of such electronic waste (E-waste) is generated which contains

considerable amount of gold. The new, cost effective and novel technology has been required for the

recovery of gold from e-waste and other secondary waste such as electroplating and leaches liquors. Bio-

adsorption has emerged as one of the low cost and often high tech option for the recovery of precious

metal ions from waste material, especially refining wastewaters. Although commercial cellulose after

cross-linking yielded very effective adsorbent for Au(III), we have tried to extent our technology for the

other low cost cellulose rich biomass thus adsorbent was prepared from cotton by the similar method of

cross-linking as in Chapter 2 and investigate its adsorption behavior at various experimental conditions.

In the present work, we have developed a novel, cost effective and highly efficient material for selective

recovery of gold using cotton cellulose. The adsorbent was found to be highly selective for Au(III) with

high adsorption capacity (6.21 mmol g-1

). It was found that the adsorbed Au(III) was reduced to elemental

form and easily recovered by incineration technique.

62

3.1 INTRODUCTION

The gold is a precious metal, greatly appreciated by mankind throughout history. Despite

attempts to replace gold in the electronics industry, demand for it has continued to grow.

Nowadays, gold is used on a massive scale in technological fields, especially computers, which

require a high degree of reliability. A new technology is required for the recovery of gold from

secondary sources such as electronic scraps, waste from electroplating baths, and leach liquors1.

Thus, it is known that gold is one of the most expensive metals reaching a high price in the

international market2. This increasing price of gold with its ever increasing demand is not only

due to the traditional jewelry values but also due to the vast application of gold in various high-

tech industries3-7

for its unique physical and chemical characteristics. In recent years, the

manufacturing of electronic and electrical devices is a major demand sector for precious metals8

like gold. The high pace of technological change and competitive market strategies that

encourage people to purchase the latest models before their old appliances stop functioning have

brought about a rapid increase in electronic and electrical wastes (e-wastes). Among metals

contained in e-wastes, gold content is sometimes unexpectedly higher than that in ores9. Over the

past decades, recovery of precious metals like gold from secondary sources such as e-wastes is

favored from environmental points of view since it can contribute to reduce the dependencies on

primary metal resources10

. Existing methods of gold recovery, such as hydrometallurgical and

chemical routes11

can be applied to various wastes but are often energy consuming (e.g.

electrolytic refining) or are environmentally unfriendly (e.g. cyanidation)12

. Various

conventional methods for recovering of gold such as chemical precipitation, carbon adsorption13

,

solvent extraction14

, reductive exchange and electrolytic recovery15

and ion exchange16, 17

have

significant disadvantages, which include incomplete metal recovery, consumption of large

63

amounts of reagents and energy, high capital costs and generation of toxic sludge or generation

of other waste products that require further purification before disposal18

. Therefore, alternative

clean recovery methods with increased efficiency are of emerging significance. Bio-sorption is

one of the low cost processes that can recover gold from dilute solutions and thus can be

considered to be suitable option for the application for the recovery of gold from industrial gold

plating waste solution and gold ore leached liquor.

In recent years, a variety of adsorbents prepared from biomass wastes and natural

products including tannin, algae, fungi and yeast biomass, alfalfa, various protein sources and

fruit wastes19-24

have been tested for the recovery of gold. However, naturally abundant and low

cost adsorption materials are still required to recover gold particularly from wastewater from

electronic and electroplating factories and leach liquor of e-wastes. Cellulose is a polydisperse

linear homopolymer consisting of regio- and enantio-selective β-1,4–glycosidic linked D-glucose

units (so called anhydroglucose units). The polymer contains three reactive hydroxyl groups at

C-2, C-3 and C-6 atoms, which are, in general, accessible to typical conversions of primary and

secondary alcoholic OH groups25

. The chemical modification of polysaccharides is the most

important route to modify the properties of the naturally occurring biopolymers and to use this

renewable resource in the context of sustainable development26

.

In the present work, gold selective adsorbent was prepared from cotton biomass by

treating with concentrated sulphuric acid for the fundamental investigation before its application

to recover gold from leach liquor of e-wastes and gold plating waste water to demonstrate its

efficiency selectivity and recover capacity by using the investigated adsorbent.

64


3.2.1 Materials and Methods

Analytical grade chloride salts of copper (Katayama Chemical, Japan), iron and

palladium (Wako, Japan) were used to prepare test solutions of respective metals. Analytical

grade of HAuCl4•4H2O and H2PtCl6•6H2O (Wako, Japan) were used to prepare gold and

platinum solutions, respectively. All other chemicals used for the preparation of adsorbent and

for adsorption tests were of analytical grade and were used without further purification.

3.2.2 Preparation of Adsorption Gel from Cotton

Cotton, Ciegal produced and marketed by Chiyoda Co. LTD., Japan, as illustrated in

Figure 3.1, was employed in the present study to prepare cotton gel as the feed material. The

chemical composition (%) of cotton is as follows: cellulose 95, hemicellulose 2, lignin 0.9 and

extract 0.4 and the remaining percentage consists of wax, ash and other organic compounds 25

.

For the preparation of the gel, 10 g of cotton was suspended in 50 cm3 of concentrated sulphuric

acid in a round bottom flask and the mixture was stirred for 24 h at 373 K for cross-linking

condensation reaction. After that, the mixture was cooled at room temperature and was

neutralized with NaHCO3, followed by washing several times with distilled water until neutral

pH. The black product obtained was dried in a convection oven for 24 h at 343 K. Then, the gel

was passed through the 75-100 µm mesh size of testing sieve for regulating the uniform particles.

The material thus obtained is known as cross-linked cotton gel and abbreviated as CLC hereafter.

65

Figure 3.1 Cotton used for preparing the adsorbent

3.2.3 Instrumental Analysis

The metal concentrations before and after adsorption were measured by using Shimadzu

model ICPS-8100 ICP/AES spectrometer and Shimadzu model AAS-6800 atomic absorption

spectrophotometer. Total organic carbon (TOC) concentration generated during the preparation

of the gel was measured by using a Shimadzu model TOC-VHS TOC analyzer. Visual

observation was recorded using KEYENCE model VHX/VH series optical microscope.

Spectroscopic studies were performed by JASCO, FT/IR-410 Fourier transform infrared

spectrometer. The formation of elemental gold, Au (0), on the gel after the adsorption of Au(III)

ion was elucidated by means of X-ray diffraction spectrum using Shimadzu model XRD-7000,

X-ray diffractometer.

3.2.4 Measurement of the Degree of Crystallinity of Cotton Cellulose

There are several ways to measure cellulose crystallinity index (CrI). One of the most

commonly employed techniques is X-ray diffraction where the peak height method is used to

calculate the CrI. However, the literature contained a wide range of reported values for native

66

cellulose using X-ray diffraction, in the range 62.0-87.6% using peak height method and from 39

to 75.3% using other methods 27

. X-ray diffraction patterns of cotton cellulose samples used in

the present study were obtained using Shimadzu model X-ray diffracto meter (XRD-7000), at

room temperature from 5 to 40º using Cu/Kα1 irradiation (1.54 Ǻ) at 40 kV and 30 mA. The

scanning speed was 2º min-1

and the data were collected in continuous mode. CrI was calculated

using the peak intensity method as proposed by Segal et al.,28

according to the following

equation.

CrI = (I002 – Iam)/I002 × 100 ………………………..(3.1)

where I002 is the intensity of the peak at 2θ = 22.5º and Iam is the minimum intensity

corresponding to the amorphous content at 2θ = 18º.

3.2.5 Adsorption Test of Various Metals Ions in Batch-wise System from Acidic Chloride

Media

In the present investigation, batch mode of operation was conducted in order to

individually measure the adsorption behavior of gold and other metal ions. Thus, in a

representative experiment, 10 mg of dried gel was shaken together with 10 cm3 of each metal ion

solution (0.2 mmol dm-3

) at varying hydrochloric acid concentration at 303 K for 24 h. After

equilibrium, the mixture was filtered and the filtrate was analyzed for remaining metal ion

concentration.

For the measurement of kinetics of adsorption, 200 mg of gel was mixed together with

200 cm3 of solution containing 2 mmol dm

-3 of Au(III) in 0.1 mol dm

-3 hydrochloric acid and

stirred by using a magnetic stirrer at 298 K. Each 3.5 cm3 solution was sampled at definite time

67

intervals from the start of the operation. Similar experiments were also carried out at 303, 313

and 323 K.

Adsorption isotherms of Au(III) were measured by shaking 10 mg (dry weight) of the gel

in 10 cm3 solutions of 0.1 mol dm

-3 hydrochloric acid varying the initial concentration of Au(III)

in the range of 0.5-12 mmol dm-3

at four different temperatures (298, 303, 313 and 323 K) for 96

h. Percentage adsorption for each metal ion was calculated according to Eq.(3.2), where Ci and

Ce (mmol dm-3

) represents the initial and equilibrium concentrations, respectively. The amount

of the adsorbed Au(III) (Q, mmol g-1

) was calculated by the mass balance calculation of Au(III)

before and after the adsorption as expressed by Eq.(3.3).

% " = �� − ��

× 100 … … … … … … … . �3.2�

� = �� − �� × � … … … … … … … … �3.3�

where V (dm3) is the volume of the test solution used and W (g) is the dry weight of the

adsorbent.

3.2.6 Thermo Gravimetric Analysis (TGA)

TGA analysis of the sample before and after loading gold was carried out using a TGA

analyzer (Shimadzu model, Simultaneous DTA-TG Apparatus, DTG-60H). Approximately 10

mg of samples before and after the gold loading was placed in a platinum (Pt) crucible for the

TGA experiment. The heating rate was maintained at 10 ºC min-1

from 30 to 800ºC.

68


3.3.1 Properties of Cotton Gel After Modification with Concentrated Sulfuric Acid

3.3.1.1 Percentage yield and TOC leak test

The product yield of the gel was calculated based on the weight of feed material

employed and it was found to be 76% in this study. The total organic carbon (TOC) generated

during the preparation was found to be 26.3 mg TOC per gram of dry raw cotton.

3.3.1.2 Studies of crystalline structure of cotton cellulose

Figure 3.2 shows the X-ray diffraction pattern of raw cotton cellulose and its cross-

linked product. The value of CrI of the former was evaluated as 78% while it decreased to 18%

after the treatment with concentrated sulphuric acid. The treatment with concentrated sulphuric

acid disrupted the crystalline structure of cotton cellulose and turned partially to amorphous

structure.

Figure 3.2 X-ray diffraction pattern of raw cotton (black multiple peaks) and cotton gel (red

single broad peak) prepared by using concentrated sulphuric acid (96%).

0

200

400

600

800

1000

1200

1400

5 15 25 35

Inte

nsit

y

2θ(degree)

69

3.3.2 Effect of Hydrochloric Acid Concentration on the Adsorption of Metal Ions

Figure 3.3 shows the % adsorption of Au(III), Pt(IV), Pd(II), Fe(III) and Cu(II) onto the

prepared gel at varying hydrochloric acid concentrations. Quantitative adsorption of Au(III) was

achieved in the low concentration range of hydrochloric acid while the adsorption extents of

other precious and base metals studied were very insignificant over the whole hydrochloric acid

concentration regions which was due to the negligible affinity of these metal ions towards cotton

adsorbent. Thus, it was evident that cross-linked cotton gel had a high affinity and selectivity for

Au(III). From the viewpoint of selectivity and extent of Au(III) adsorption, the obtained result

was quite interesting and it was expected to selectively recover Au(III) separated from a number

of other precious and base metals as tested in the present work.

Figure 3.3 Adsorption of metals ions on cotton gel as a function of hydrochloric acid

concentration. Conditions: weight of gel = 10 mg, feed solution = 10 cm3, concentration of

metals = 0.2 mmol dm-3

, shaking time = 24 h, temperature = 303 K

0

20

40

60

80

100

0 1 2 3 4 5

Percen

tag

e , %

[HCl]/mol dm-3

Au (III)

Pt (IV)

Pd(II)

Fe (III)

Cu(II)

70

3.3.3 Adsorption Kinetics Behavior of Au(III) onto Cotton Adsorbent

The adsorbent was selective only for Au(III) ions, further experimental works were

carried out only for Au(III) ions. The data of adsorption kinetics of Au(III) is illustrated in

Figure 3.4. Figure 3.4 (a) shows the variation of the adsorbed Au(III) amount on the adsorbent

as the function of shaking time at different temperature. It was observed that temperature had a

significant effect on the adsorption rate of Au(III). The time required to reach adsorption

equilibrium was shortened with increasing temperature. Furthermore, it was seen that the

adsorption rapidly increased at the initial stages and then the adsorption rate was slowed down to

zero at the final stages for all the temperatures tested i.e. 100% adsorption was achieved. Since

the equilibrium was reached nearly within 30 h for all temperatures studied for the present gel,

the contact times was fixed at 96 h in the following experiments in order to completely ensure

the adsorption equilibrium.

The kinetic data at the initial stage were re-plotted on the basis of the pseudo-first order

kinetic model according to Eq. (3.4) as shown in Figure 3.4 (b). In this figure, nearly all plots

appear to cluster on the straight lines passing through the origin corresponding to different

temperatures. From the slopes of these straight lines, the pseudo-first order rate constants were

evaluated at four different temperatures.

lnCt/Ci = - kt………………………………………….….(3.4)

where Ci and Ct (mmol dm-3

) represent the initial Au(III) concentration and the concentration at

time t after adsorption onto CLC gel, respectively, k represents the pseudo-first order rate

constant (h-1

) and t is shaking time (h).

71

In order to evaluate the activation energy of the present adsorption reaction, the pseudo-

first order rate constant evaluated at different temperatures were plotted according to the

Arrhenius equation as shown in Eq. (3.5).

lnk = lnA - Ea/RT ………………………….…………. (3.5)

where A represents the frequency factor, R is the universal gas constant, Ea is the activation

energy (kJ mol-1

) and T is the absolute temperature (K).

Figure 3.4 (c) depicts the relationships between the rate constants and temperature

according to Eq. (3.5). The activation energy (Ea) was evaluated from the slope of the straight

line in this figure as 78.8 kJ mol-1

which is nearly close (81.7 kJ mol-1

) to the value evaluated for

persimmon peel gel evaluated earlier from our group29

. In both the cases the high value of

activation energy suggests that adsorption rate is controlled by chemical reaction.

0

0.5

1

1.5

2

2.5

0 20 40 60 80

q [

mm

ol/

g]

Time [h]

323 K

313 K

303 K

298 K

(a)

q/m

mo

lg

-1

t/h

-4.5

-3.7

-2.9

-2.1

-1.3

-0.50 20 40 60

ln C

t/C

i

Time [h]

323 K

313 K

303 K

298 K

(b)

t/h

72

Figure 3.4 Adsorption rate of Au(III) by the cotton gel at different temperatures. (a)

experimental plot, (b) pseudo first order plot and (c) Arrhenius plot. Conditions: weight of the

dry gel = 200 mg, volume of the solution = 200 cm3, [Au(III)] = 2 mmol dm

-3, [HCl] = 0.1 mol

dm-3

3.3.4 Adsorption Isotherms and Thermodynamic Investigation

The adsorption isotherms of Au(III) onto the cross-linked cotton gel at four different

temperatures are shown in Figure 3.5 (a). The amount of adsorbed Au(III) significantly

increased with the increase in Au(III) concentration at low concentration while it gradually

tended to approach constant values corresponding to temperatures at higher Au(III)

concentrations, which appeared to be in accordance with the (well-known) Langmuir adsorption

isotherm. Furthermore, the increase of temperature facilitated the adsorption of Au(III).

Consequently, the adsorption isotherms data were analyzed in terms of the Langmuir

adsorption model which was based on the assumption that the adsorption took place on identical

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

3 3.1 3.2 3.3 3.4ln

k

1/T ×103 [K-1]

(c)

73

adsorption sites on a homogenous monolayer surface and there is no lateral interaction between

the adsorbed sites as shown in Eq. (3.6).

��

= 1��.� + ��

��… … … … … … … . … … … … … … . �3.6�

where Qe (mmol g-1

) represents the equilibrium adsorption amount, Qmax (mmol g-1

) represents

the maximum adsorption capacity, b (dm3 mmol

-1) is the Langmuir constant or adsorption

equilibrium constant related to the adsorption energy.

Table 3.1 Thermodynamic parameters for the adsorption of Au(III) on acid treated cotton

Temp.

(K)

b

(dm3 mmol

-1)

lnb

q

(mmol g-1

)

�G°

(kJ mol-1

)

�H°

(kJ mol-1

)

�S°

(J mol-1

K-1

)

R2

298 31.53 3.45 5.29 -8.54

142.50

33.63

0.996

303 53.67 3.98 6.21 -10.03 0.999

313 63.50 4.15 7.87 -10.80 0.999

323 102.0 4.62 9.80 -12.42 0.999

The result showed that experimental data were in good agreement with the Langmuir

isotherm with a high correlation coefficient as shown in Figure 3.5 (b). The values of Qmax and b

were evaluated from the slope and intercept of the straight lines in this figure and listed in Table

3.1. The values of Qmax and b were found to be increased with increasing temperature of the

system which is one of the indication that adsorption of Au(III) onto CLC was endothermic in

nature which was further proved by calculating thermodynamic parameters.

74

High value of the Langmuir constant (b) reflected a strong bonding of Au(III) ions with

the adsorbent30

. Comparison of maximum adsorption capacities (Qmax) among various adsorbents

is listed in Table 3.2. It demonstrated that the cotton gel prepared in this study exhibited the

highest adsorption capacity compared to other adsorbents, suggesting the promising application

of the present gel for Au(III) recovery from aqueous solution.

Table 3.2

Comparison of maximum adsorption capacity for Au(III) among various adsorbents

Adsorbents Qmax Adsorption media References

(mmol g-1

)

Cross-linked cotton gel 6.21 0.1 M HCl Present study


Dimethyl amine paper gel 4.6 1 M HCl [32]

Cross-linked lignophenol (CLP) 1.92 0.5 M HCl [33]

Primary amine-CLP 1.98 0.5 M HCl [33]

Ethylene diamine-CLP 3.08 0.5 M HCl [33]


Rice husk carbon 0.76 1 M HCl [34]

Barley straw carbon 1.47 1 M HCl [34]

Alginate cross-linked with CaCl2 1.47 pH 2 [1]

From the evaluated values of b at varying temperature, some thermodynamic parameters,

changes in the free energy (∆G°), enthalpy (∆H°) and entropy (∆S°) associated to the adsorption

process were evaluated according to the following Eqs. (3.7) and (3.8).

75

∆G° = - RT lnb……………………………………………………….(3.7)

lnb = - ∆G°/RT = - ∆H°/RT + ∆S°/R ……………………………… .(3.8)

The plot lnb versus 1/T as shown in Figure 3.5 (c) yielded a straight line with the

correlation coefficient (r2) of >0.94, from which the values of ∆H° and ∆S° can be calculated

from slope and intercept, respectively. The negative values of Gibbs free energy (∆G°) at all

temperatures studied, confirm the feasibility and thermodynamically favorable process and

spontaneous nature of the adsorption as listed in Table 3.1. The more negative of ∆G°, the

stronger the driving force of the adsorption reaction. The decrease in the value of ∆G° with the

increase of temperature shows that the reaction is more spontaneous at a high temperature which

indicates that the adsorption processes are favored by the increase in temperature. Positive value

of enthalpy (∆H°) demonstrates the endothermic nature of the adsorption. Entropy, as the

measure of randomness of the system, also indicates the positive values suggesting the increased

randomness at solid-solution interface during the adsorption of Au(III).

76

Figure 3.5 Adsorption isotherms of Au(III) onto cotton gel at different temperature. (a)

Experimental plot, (b) Langmuir plot and (c) van’t Hoff plot. Conditions: weight of dry gel = 10

mg, volume of solution = 10 cm3, shaking time = 96 h, [HCl] = 0.1 mol dm

-3

2

3

4

5

3 3.1 3.2 3.3 3.4

ln b

1/T ×103 [K-1]

(c)

0

2

4

6

8

10

12

0 2 4 6 8

q [

mm

ol/

g]

Ce [mmol/L]

323 K

313 K

303 K

298 K

(a)

q/m

molg

-1

Ce/mmol dm-3

0

0.5

1

1.5

0 2 4 6 8

Ce/

qe[g

/L]

Ce [mmol/L]

323 K

313 K

303 K

298 K

(b)

Ceq

-1/g

dm

-3

Ce/mmol dm-3

77

3.3.5 Solid State Analysis of Cotton Gel after Contacting with Au(III) Solution

Figure 3.6 shows the XRD pattern of the gel after the adsorption of Au(III) from 10 to 80

degree. The scanning speed was 2 degree min-1

and data were taken by continuous mode. It was

observed that the intensive characteristic peaks of elemental gold were located at the 2θ values of

38.08, 44.34, 64.56 and 77.48. This result confirmed that the adsorption of Au(III) was

accompanied by the formation of elemental gold, which was further confirmed by optical

microscope as shown in Figure 3.7(b). Figure 3.7(a) shows a photograph of the sample solution

and the gel in a glass vessel after shaking. The aggregated gold particles were floating on the

surface of the sample solution of acidic media. Figure 3.7(b) shows the aggregated particles of

gold after the adsorption on the surface of cotton gel with various shapes. Similar phenomenon

was reported for the adsorption of Au(III) on the gel prepared from persimmon peel waste in the

similar manner using sulphuric acid 29

.

Figure 3.6 XRD patterns of cotton gel after the adsorption of Au(III) by CLC gel

0

1000

2000

3000

4000

5000

6000

10 20 30 40 50 60 70 80

Inte

nsi

ty (co

un

ts)

2θ

38.08

44.34

64.56 77.48

78

Extremely high selectivity and loading capacity of the present gel for Au(III) may be

attributed to the reduction of Au(III) to Au(0) during the adsorption. These results proved that

gold particles formed on gel surface by the reduction process exhibited a certain sort of tendency

to be detached from the gel surface forming clear aggregates. A similar observation was also

found in our previous study 32

.

Figure 3.7 Optical microscope photograph of gold aggregates (a) floating on the surface of

sample solution and (b) dried gel, after adsorption of Au(III)

3.3.6 Fourier Transformed Infrared Spectroscopy Studies

In order to elucidate the mechanism of Au(III) adsorption on the cotton gel, the

characterization of raw cotton as well as the cotton gel before and after Au(III) adsorption were

investigated by means of FT-IR spectroscopy as illustrated in Figure 3.8. The spectra showed

that the broad peak appeared at 3421 cm-1

in raw cotton was derived from O-H stretching band

and the peak at 2912 cm-1

was assigned to the C-H stretching vibration of cotton cellulose. The

(b)(a)

79

peak at 1647 cm-1

is due to C=O stretching vibration of carbonyl group of cotton cellulose. After

acid treatment, the peak intensities at 3421 cm-1

and 2912 cm-1

decreased, this was attributable to

participation of some hydroxyl group for cross-linking reaction. Furthermore, a new peak

appeared at 1192 cm-1

which was attributed to the formation of C-O-C linkages, which was one

of the strong evidence of the condensation reaction by acid treatment. The O-H stretching

vibration at 3389 cm-1

in cotton gel became broader and shifted to 3325 cm-1

after the adsorption

of Au(III) ion which was attributed to the coordination of adsorbed Au(III) ion with hydroxyl

group because metal coordination weakened the O-H bond and absorption shifted towards the

lower frequency region. Similarly, the intensity of peak at 1699 cm-1

, increased after Au(III)

adsorption. It was reasonably explained to be attributable to the oxidation of the hydroxyl groups

to carbonyl group in the cotton gel; i.e. such carbonyl group may be resulted from the Au(III)

reduction.

Figure 3.8 FT-IR spectra of (a) raw cotton (b) cross-linked cotton (CLC) gel and (c) Au(III)-

loaded cross-linked cotton (Au-CLC) gel

400100016002200280034004000

Tra

nsm

ita

nce [

%]

Wavenumber [cm-1]

(a)

(b)

(c)

80

3.3.7 Thermo-gravimetric Analysis for the Recovery of Gold in Its Elemental Form

After long time contact with Au(III) solution, cotton gel adsorbed Au(III), which was

then reduced to metallic gold particles. Thus, adsorbed gold needed suitable and cost effective

technology for its recovery process. Incineration was one of the favorable options to recover

metallic gold from gold loaded cotton gel in simple manner. Lines (a) and (b) in Figure 3.9

show the change in weight percentage of the gold-loaded and un-loaded cotton gels by

temperature, respectively. The results revealed that about 10 % of the weight was observed to

remain after the incineration of the unloaded gel whereas, in the case of the gold-loaded gel,

about 20% of weight remained after the incineration. This difference in the weight corresponded

to that of the loaded gold in CLC at mentioned experimental condition.

Figure 3.9 Thermo-gravimetric curves of (a) gold-loaded cotton gel, (b) cotton gel before the

adsorption of Au(III) and (c) image of optical microscope of the gold-loaded cross-linked cotton

gel after incineration.

81

3.3.8 Mechanism of Au (III) Adsorption-Reduction onto Cotton Adsorbent

The mechanism by which the adsorbent binds metal ions has recently received increased

attention as the advantages of understanding such mechanisms become more apparent. Broadly,

adsorption can be divided into chemical and physical adsorption mechanisms. Chemical

mechanism includes chelation, micro-precipitation and micro-reduction, while physical

mechanism generally involved electrostatic forces (18,

35

).

It was inferred from the IR spectra presented in Figure 3.8 that the polymeric

structure of cotton cellulose upon the treatment with concentrated sulphuric acid underwent

cross-linking condensation reaction with the release of water molecules forming new linkage

by ether bonds suitable for Au(III) binding as shown in Scheme 3.1. The adsorbent prepared

by cross-linking with concentrated sulphuric acid facilitated not only the dehydration of the

cellulosic material but also the suitable configuration for binding an Au(III) ion during

coordination reaction. The Au(III) formed anionic tetrachloroaurate (AuCl4-) over a wide

concentration range of hydrochloric acid and was coordinated to the oxygen atom of C-O-C

linkage formed by the acid treatment and to the oxygen atoms of the C2 and C3 hydroxyl

groups of pyranose ring as depicted in Scheme 3.1. In the IR spectrum, OH stretching

vibration at 3389 cm-1

after suphuric acid treatment became broader and shifted to low

frequency (3325 cm-1

) region after Au(III) adsorption which was caused by the coordination

of adsorbed Au(III) with hydroxyl groups. After Au(III) adsorption, the hydroxyl groups

coordinated to Au(III) were oxidized to carbonyl groups, providing electrons necessary for

Au(III) reduction into elemental gold, Au (0). Here, it should be noted that Au(III) has a high

redox potential (1.40 V) in comparison to other metal ions like Pd2+

(0.68 V) and Pt4+

(0.62

82

V). It was inferred that these were the driving force for the formation of gold particles,

resulting in the extraordinary high selectivity and loading capacity.

O

H

H

H

OH

O

H

H

H

OO

O Au

O

O

H

Cl

Cl

Cl

H

O

n

m

O

H

H

H

O

H

H

H

OO

O

O

H

H

O

n

m

O

O

CH2OH CH2OH

CH2OH CH2OH

H

O

H

H

H

H

O

H

H

H

H

OO

O

O

H

H

O

n

m

CH2OH

CH2OH

OH

OH

O

H

H

H

H

O

HO

n

CH2OH

OHOH

O

H

H

H

H

OO

Hm

CH2OH

OHOH

+

Cotton cellulose

H2SO4 cross-linking

H2O

AuC

l 4- ads

orpt

ion

Au(III) reduction+ Au0 +4H+

+ 4Cl-

Cross -linked cotton gel

Au(III) adsorbed cotton gel Formation of elemental gold on cotton gel

Scheme 3.1 Synthetic route of cross-linked cotton gel and mechanism of Au(III)

adsorption followed by reduction

83

3.3.9 Recovery of Gold from the Leach Liquor of Spent Mobile Phones

After the printed circuit boards taken from spent mobile phones (Shibata Industry Co.Ltd.

Omuta, Japan), were mechanically crushed and calcined at 1023 K for 6 h and (Shonan Factory

of Tanaka Kikinzoku Kogyo K.K., Hiratsuka, Japan) the powdered sample was leached at first

with nitric acid to remove the majority of silver and base metals, and then with chlorine

containing hydrochloric acid solution, from which gold was recovered. The metal contents in the

latter liquor were as follows (unit: mg dm-3

). Au 170, Pt 380, Pd 30, Ag 60, Fe 3500, Cr 600, Ni

400, Cu 300, Sn 200, Al 265.

Figure 3.10 shows the adsorption of various precious and base metals from the actual

leach liquor of chlorine containing hydrochloric acid solution for circuit board of spent mobile

phones using cross-linked cotton (CLC) gel at varying solid/liquid ratio (= dry weight of the

adsorbent added/volume of leach liquor). It was apparent that CLC gel exhibited high selectivity

for gold (III) also from the actual solution containing large numbers of co-existing precious and

base metals.

84

Figure 3.10 Effect of solid/liquid ratio on the % adsorption of various metals from leach liquor

of chlorine containing hydrochloric acid solution for circuit board of spent of mobile phones

using cross linked cotton gel. Condition: volume of solution: 10 cm3, shaking time = 48 h,

shaking speed = 150 rpm, temperature = 303 K

3.4 CONCLUSIONS

The interaction of the acid treated cotton with Au(III) has been investigated in terms of

adsorption tests, IR spectroscopy and XRD analysis, from which it was suggested that the

Au(III) adsorption involved an apparent redox reaction forming aggregates of gold particles; that

is, the metal ion was reduced to metallic form during adsorption. The acid-treated adsorbent had

a remarkably high affinity for the gold ions due to the –C-O-C- ether groups provided by the acid

treatment and hydroxyl groups on the surface of the cotton gel. The hydroxyl groups of the

adsorbent played a leading role on the Au(III) binding followed by the reduction. The XRD

pattern and optical photograph of cotton gel after the adsorption of Au(III) supported the

0

20

40

60

80

100

0 2 4 6 8 10

% A

dso

rpti

on

of

me

tal

ion

s

Adsorbent dose (g dm-3)

Au(III)

Pt(IV)

Pd(II)

Cu(II)

Ni(II)

Fe(II)

85

reduction of Au(III) to very fine particles of elemental gold. The very high adsorption capacity

and selectivity of cotton gel to Au(III) was attributable to the change in the structure of the

polymer matrices of crystalline cellulose by the acid treatment. The incineration method was the

suitable way to recover gold from gold adsorbed gel in its metallic form. Thus, the present work

suggested the possibility of new uses of cotton fiber materials including spent cotton wears for

the recovery of gold from various waste, especially from e-wastes from hydrochloric acid media.

86

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19. .Gomes CP, Almeida MF, Lourerio JM (2001) Gold recovery with ion exchange used

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91


An Assessment of Gold Recovery Processes Using Cross-Linked Paper Gel

The cross-linked cotton (CLC) gel prepared from cotton cellulose in Chapter 3 was effective to

recovered Au(III) into elemental form in hydrochloric acid media. We were intended to elaborate our

technology to the cellulose rich paper biomass because Japan is the third largest country for paper

production in the world thus raw paper for adsorbent preparation was easily available at low cost in Japan.

The paper was first cut into small pieces and cross-linked with concentrated sulphuric acid according to

the method described in Chapters 2 and 3. The cross-linked paper (CLP) gel prepared in this study also

showed very good affinity for Au(III). The adsorbed gold was simultaneously reduced to elemental form

which was elucidated from X-ray diffraction spectrum, SEM，and an optical microscope. The inferred

mechanism for the adsorption of Au(III), followed by its reduction to elemental gold, was proposed based

on the observation of infrared spectroscopic analysis. It was inferred that Au(III) was adsorbed by the

coordination to oxygen atom of C-O-C linkage formed after the cross-linking with concentrated sulphuric

acid, as well as the oxygen atoms of hydroxyl groups of pyranose rings of paper cellulose. The adsorbed

gold can easily be recovered by incineration method at elevated temperature.

4.1 INTRODUCTION

During the past decades, with the development of consumer-oriented electrical and

electronic technologies, large amounts of electronic equipment have been provided to the market.

The useful life of these devices is relatively short and has been decreasing as a result of rapid

92

changes in equipment features and capabilities. The short useful life of these products brings

about a large waste stream of obsolete electronic and electrical devices (e-waste).

Precious metals have a wide variety of applications in the manufacturing of electronic

appliances, serving as contact materials due to their high chemical stability and good conducting

properties1. Gold recovery from secondary sources such as electronic scraps and waste

electroplating solutions is therefore, remarkable technology2. Several studies reported that

recovering precious metals from electronic scraps is one of the greatest economic profits for the

recycling industry. The disposal e-wastes contain higher proportion of precious metals than in

the ore itself. Additionally, the purity of precious metals like gold in the printed circuit board

(PCB) in e-waste is more than 10 times that of the corresponding minerals3.

Many processes such as solvent extraction, ion exchange, and co-precipitation have been

available to separate and enrich the gold4-6

. However these processes are only feasible to

specially processed solutions containing high gold concentrations. These processes are also

suffered from various problems like low yields, operational difficulties, and environmental

concerns. On the other hand, adsorptive separation, which is an efficient recovery process for

gold from dilute solutions, also suffers from some problems such as insufficient selectivity. This

gives a compelling reason for developing more efficient and more environmental-friendly

methods for the selective recovery of gold, not only from mineral ores but also from waste

materials like e-waste7. Bio-adsorption is expected to be a promising technique for metal

recovery since biomasses not only exhibit effective adsorption behavior for metal ions, but also

is environmental-friendly, biodegradable, and biocompatible. Thus, they have attracted

significant attention in recent years.

93

Cellulose, a polysaccharide made by most plants, is one of the most abundant organic

compounds on earth. The chemical modification of polysaccharides is the most important route

to modify the properties of the naturally occurring biopolymers, including cellulose, and to use

this renewable resource in the context of sustainable development. This is because the hydroxyl

groups in cellulose can coordinate with metal ions to form stable complexes. Although, different

kinds of adsorbents such as dimethylamine-immobilized (DMA) paper, p-aminobenzoic (PAB)

acid modified paper, and iminodiacetic acid (IDA) type paper were identified earlier to recovered

Au(III) from trace concentration8-10

but high costs were focused on the preparation of these

adsorption gels from spent paper, one of cellulosic materials, though the feed material was cheap.

In this chapter, we attempted to develop more efficient adsorbent for effective recovery of

Au(III) by means of a much more simple method at cheap cost from paper cellulose.


4.2.1 Materials and Method

Advantec, Quantitative Ashless (5C, 150 mm) filter paper produced by Toyo Roshi Co.,

Ltd. Japan, was used as the feed material for the preparation of adsorption gel. Analytical grade

chloride salts of zinc (Sigma Aldrich), iron, palladium (Wako, Japan), and copper (Katayama

Chemical, Japan) were used to prepare the test solutions of respective metals. Analytical grade

HAuCl4•4H2O and H2PtCl6•6H2O (Wako, Japan) were used to prepare gold and platinum

solutions, respectively. All other chemical reagents used for the preparation of adsorbent and for

the adsorption tests were of analytical grade and used without further purification unless

mentioned.

94

4.2.2 Preparation of Adsorption Gel

For the preparation of adsorption gel, first raw filter paper was cut into small pieces and

10 g of paper was mixed together with 50 cm3 of 96% concentrated sulphuric acid (Wako, Japan)

in a round bottom flask, the mixture was continuously refluxed for 24 h at 373 K to enhance the

cross-linking condensation reaction. During this condensation reaction, some of the hydroxyl

groups in polymer chains of paper cellulose (shown in Figure 4.1) were condensed to form ether

bonds as shown in Scheme 4.1. Next, the mixture was cooled in room temperature and was

neutralized with sodium bicarbonate. The insoluble cross-linked paper product was collected by

vacuum filtration and washed several times with distilled water followed by hot water in order to

remove the excessive acid from the gel. The gel was then dried in a convection oven for 24 h at

343 K. Then, the dried gel was sieved to produce a particle size fraction between 75-100 µm; this

is abbreviated as cross-linked paper (CLP) gel hereafter.

O

H

H

H

H

O

HO

n

CH2OH

OHOH

Figure 4.1 Structure of paper cellulose

95

O

H

H

H

O

H

H

H

H

OO

O

O

H

H

O

n

m

CH2OH

CH2OH

OH

OH

O

H

H

H

O

HO

n

CH2OH

OHOH

O

H

H

H

H

OO

Hm

CH2OH

OHOH

+

Paper cellulose

H2O

Cross linked paper cellulose

1000C

HH

H2SO4 ,

Scheme 4.1 Preparation of cross-linked paper (CLP) adsorbent

4.2.3 Measurement and Analysis

The metal concentrations before and after the adsorption were analyzed by using

Shimadzu model ICPS-8100 ICP-AES spectrometer and Shimadzu model AAS-6800 atomic

absorption spectrophotometer. Total organic carbon (TOC) concentration generated during the

preparation of the gel was measured by using a Shimadzu model TOC-VHS TOC analyzer. The

scanning electron microscopy (SEM) analysis of the adsorbents was performed using the JEOL

model JSM 5200 scanning microscope. Visual observation of elemental gold was recorded using

KEYENCE model VHX/VH series optical microscope. Spectroscopic studies were performed by

JASCO model FT/IR-410 Fourier transform infrared spectrometer. The generation of elemental

gold (Au0) on the gels after the adsorption of Au(III) was elucidated by means of X-ray

diffraction spectrum using Shimadzu model, XRD-7000, X-ray diffractometer. Thermo

96

gravimetric analysis (TGA) of the sample before and after adsorption of gold was carried out

using a TGA analyzer (Shimadzu model, Simultaneous DTA-TG Apparatus, DTG-60H).

4.2.4 Batch-wise Adsorption Tests

The adsorption behavior of the cross-linked paper gel for Au(III), Pd(II), Pt(IV), Fe(III),

Cu(II) and Zn(II) were individually examined at varying hydrochloric acid concentrations. Each

metal (0.2 mmol dm-3

) solution was prepared by varying the concentration of hydrochloric acid

from 0.1 to 5 mol dm-3

. In typical runs, 10 mg of dried gel (CLP) was shaken together with 10

cm3 of each metal solution using a thermostated shaker maintained at 303 K for 24 h to attain

equilibrium, which will be described later. After shaking, the mixture was filtered using

Advantec filter paper (5C, 90 mm) and the filtrate was analyzed for residual metal concentrations.

Adsorption isotherms of Au(III) on CLP gel was examined to evaluate the adsorption

capacity of the adsorbent at four different temperature (298, 303, 313 and 323 K). A number of

Au(III) solutions containing varying concentrations from 0.1 to 16 mmol dm-3

were prepared in

0.1 mol dm-3

hydrochloric acid solutions. Then, 10 cm3 aliquots of these solutions were mixed

together with 10 mg of dried CLP gel and were shaken at the four different temperatures for 96 h

to ensure complete equilibrium. All adsorption tests were repeated at least twice, and the

obtained results reproduce the negligible differences.

Adsorption kinetic studies were conducted using 200 cm3 (2 mM i.e. mM = mmol dm

-3)

of Au(III) solution (which was prepared in 0.1 mol dm-3

hydrochloric acid solution) together with

200 mg of the dried CLP gel at four different temperature (303, 308, 313 and 323 K), at varying

contact times, to measure the adsorption rates at different temperatures and evaluate the apparent

activation energy. The sample solution was taken at definite time intervals and analyzed for

97

remaining Au(III) concentration after adsorption. The percentage adsorption of metal ions was

calculated according to Eq. (4.1).

% "%&'()*+', = �� − ��

× 100 … … … … … … … . �4.1�

where Ci and Ce (mmol dm-3

) are the initial and equilibrium concentrations of metals ions,

respectively.

4.2.5 Thermo Gravimetric Analysis (TGA)

TGA analysis of the sample before and after loading gold was carried out using the TGA

analyzer. Approximate 10 mg of samples before and after the gold loading was placed in

platinum (Pt) crucible for the TGA measurement. The heating rate was maintained at 10 ºC min-1

from 30 to 1000ºC.


4. 3.1 Product Yield and TOC Leak of CLP gel

The product yield of CLP gel was evaluated to be 73% based on the weight of feed

material employed according to Eq. (4.2). The total organic carbon (TOC) leaked during the

preparation of CLP gel, after washing with distilled water, was evaluated to be 26.2 mg of TOC

g-1

of dry raw paper.

% -+./% = ��0 − �12��0

× 100 … … … … … … … . �4.2�

where Wr and Wcl (g) are the weights of raw paper and CLP gel, respectively.

98

4.3.2 Effect of Hydrochloric Acid Concentration on the Adsorption of Metal Ions

The adsorption behavior of Au(III), Pt(IV), Pd(II), Zn(II), Fe(III), and Cu(II) on CLP gel

at varying hydrochloric acid concentrations (0.1 – 5 mol dm-3

), which was tested individually is

presented in Figure 4.2. CLP gel was found to be selective only for Au(III) and a significant

extent of adsorption i.e. almost 100% adsorption was achieved over a wide range of the

hydrochloric acid from 0.1- 4 mol dm-3

while the % adsorption of Au(III) was found to be

lowered with further increasing the acid concentration beyond 4 mol dm-3

, the reason of which

will be discussed in detail later in section 3.8. Such selective adsorption of Au(III) on CLP gel

was quite interesting because the gel was almost completely inert not only towards coexisting

base metals such as cupric, ferric, and zinc ions, but also towards coexisting other precious

metals such as Pt(IV) and Pd(II) ions. This meant that CLP gel prepared by a very simple method

has the highest affinity for Au(III). To ensure the effect of other metal ions on the adsorption of

Au(III), the mixture solution of Au(III), Cu(II), Fe(III), Pd(II), Pt(IV) and Zn(II) at 0.2 mmol dm-

3 at varying hydrochloric acid concentration were prepared to conduct the adsorption tests in a

typical coexisting system. The results obtained were exactly the same as that for individual

system i.e., similar adsorption was observed for Au(III) and negligible adsorption was observed

towards other precious and base metals tested in the present study. On the basis of these results,

further experiments were conducted only for Au(III).

99

Figure 4.2 Adsorption of various metals by cross-linked paper gel at varying hydrochloric acid

concentrations. Conditions: metal concentrations = 0.2 mmol dm-3

, volume of solution = 10 cm3,

weight of gel = 10 mg, shaking time = 24 h, temperature = 303 K.

4.3.3 Adsorption Kinetic Studies

The rate of Au(III) adsorption on CLP gel was observed by varying the contact times at

four different temperature (303, 308, 313, and 323 K). The initial Au(III) concentration was 2

mmol dm-3

, which was prepared in 0.1 mol dm-3

hydrochloric acid solution. A 200 mg of dried

gel was shaken together with 200 cm3 of Au(III) solution at different time intervals, at which 3.5

cm3 aliquot was sampled, and immediately filtered. The filtrate was analyzed for residual Au(III)

concentration, from which the amount of Au(III) adsorption was calculated at each time. Figure

4.3 (a) shows the time variation of the adsorbed Au(III) amount at each temperature. It was clear

that the adsorption increased with increasing the agitation time and the equilibrium approached

within 4 h at 323 K, 8 h at 313 K, 28 h at 308 K and 32 h at 303 K. Thus, the equilibrium time

was increased with decreasing the temperature under such conditions. Hence, in the

measurement of adsorption isotherms, the mixtures were shaken for 96 h to ensure the complete

0

20

40

60

80

100

0 1 2 3 4 5

Pe

rce

nta

ge

, %

[HCl]/mol dm-3

Au

Pt

Pd

Fe

Cu

100

equilibrium. Also, the amount of the adsorbed Au(III) was found to increase with increasing

temperature of the adsorption system. At different times, the amount of Au(III) adsorbed (qt,

mmol g-1

) on CLP gel was calculated from a mass balance by Eq.(4.3).

34 = �� − �4�� × � … … … … … … … … … . . … … … … … . �4.3�

where Ci and Ct are the initial and residual concentration of Au(III) in the solution (mmol dm-3

),

respectively. V(dm3) is the volume of Au(III) solution and W(g) is the dry mass of CP gel used.

In this study, the kinetic data were analyzed on the basis of the Lagergren pseudo-first

order reaction model as expressed by Eq. (4.4) to investigate the adsorption rate and activation

parameter of the Au(III) adsorption on CLP gel.

/, 5�4��

6 = −7* … … … … … … … … … … … . … … … . �4.4�

where, Ci and Ct (mmol dm-3

)are the initial concentration and concentration of Au(III) at time t

(h), k is the pseudo-first order rate constant (h-1

).

The plot of ln(Ct/Ci) versus time(t) is shown in Figure 4.3 (b). All the plots for each

temperature studied were found to lie on straight lines passing through the origin. From the

slopes of these straight lines, the rate constants were evaluated for each temperature. The pseudo-

first order rate constants and corresponding correlations coefficients for four different

temperatures are listed in Table 4.1. The values of rate constants increased with increasing

temperatures. Similar results were observed for the Au(III) adsorption on cross-linked grape

waste gel11

. The rate constant, k, at the different temperature (303, 308, 313 and 323 K) listed in

Table 4.1 were used to calculate the activation energy for the present adsorption reaction

according to the Arrhenius Eq.(4.5).

101

/,7 = /," − 89:T … … … … … … … … … … … … … … … … . �4.5�

where R is the gas constant (8.314 kJ mol-1

).

Table 4.1 First order rate constants and corresponding correlation coefficient for the

adsorption of Au(III) on cross-linked paper gel at different temperatures

Temp (K) Rate constant (h-1

) R2

303 0.211 0.996

308 0.355 0.939

313 0.597 0.985

323 0.959 0.992

q/m

molg

-1

t/h

t/h

102

Figure 4.3 Adsorption kinetics of Au(III) by concentrated sulphuric acid cross-linked paper gel

at different temperature (a) experimental plot, (b) pseudo-first order plot, (c) Arrhenius plot.

Conditions: volume of solution = 200 cm3, weight of the dry gel added = 200 mg, concentration

of Au (III) = 2 mmol dm-3

, [HCl] = 0.1 mol dm-3

Figure 4.3 (c) shows the plot of lnk versus T-1

. As expected from Eq. (4.5), the plots lie

on a straight line in this figure. From the slope of this straight line, the activation energy (Ea) was

estimated as 61.3 kJ mol-1

. This value suggests that the adsorption of Au(III) on to CLP gel is a

chemical adsorption, which was confirmed from the fact that the values of Ea for chemical

adsorption is usually between 8.40 and 83.7 kJ mol-1

. The positive value of Ea suggests that a

increase in temperature favors adsorption of Au(III) by CLP gel. This behavior of adsorption of

Au(III) by CLP gel was also supported by the value obtained from thermodynamic parameters i.e.

change in enthalpy which indicated a chemisorption process to be endothermic in nature (as

described in detail later).

103

4.3.4 Adsorption Isotherms and Thermodynamic Investigation

Figure 4.4 (a) shows the adsorption isotherms of Au(III) at four different temperature

(298, 303, 313 and 323 K). It is clear that the concentration of Au(III) in the low concentration

region, increased with significant increasing the amount adsorption of Au(III). This figure also

shows the plateau regions, observed at high concentration region for each temperature,

suggesting that the Langmuir-type adsorption took places under the present experimental

conditions. The result shows that the temperature has great effect for the adsorption of Au(III) on

CLP gel. All the results of isotherm studies were re-plotted according to Eq. (4.6) on the basis of

the Langmuir model as shown in Figure 4.4 (b), where all plots lie on straight lines

corresponding to different temperature.

max

e

maxe

e

Q

C

bQ

1

Q

C+= …………….………………. (4.6)

where Ce(mmol dm-3

) is the concentration of Au(III) after adsorption, Qe(mmol g-1

) represents

the equilibrium adsorption capacity, Qmax(mmol g-1

) represents the maximum adsorption capacity,

b (dm3 mmol

-1) is the Langmuir constant or equilibrium constant related to the adsorption energy.

From the Figure 4.4 (b), the maximum adsorption capacities (Qmax) and Langmuir or

equilibrium constant (b) were calculated from the slopes and intercepts respectively, for each

temperature studied and the evaluated values are listed in Table 4.2. It was obvious that with

increasing temperature of the system, the maximum adsorption capacity was increased, this

suggested the endothermic nature of adsorption reaction of Au(III) on CLP gel.

Langmuir constant (b) served as an indicator which quantitatively reflected the affinity

between the adsorbent and the adsorbate13

. The applicability of the Langmuir constant or

equilibrium constant (b) can be better understood for the determination of thermodynamic

parameters. Different thermodynamic parameters such as change in Gibbs free energy (∆Gº),

104

change in enthalpy (∆Hº), and change in entropy (∆Sº) can be calculated using the following

Eqs.(4.7) and (4.8).

∆Gº = -RT lnb……………….…………………..…………(4.7)

∆Gº = ∆Hº - T∆Sº…………………………….…..………..(4.8)

From Eqs. (4.7) and (4.8), van’t Hoff equation is expressed by Eq.(4.9).

/,� = ∆>°

: − ∆?°

:@ … … … … … … … … … … … … . … … … … . �4.9�

As shown in Figure 4.4 (c), the plot of lnb versus T-1

gives a straight line with a slope of

(-∆Hº/R) (kJ mol-1

) and an intercept of ∆Sº/R (kJ mol-1

K-1

).The evaluated values of

thermodynamic parameters are also listed in Table 4.2. The negative increasing values of change

in Gibbs free energy (∆Gº) with increasing temperatures indicated an increase in the feasibility

and spontaneity of the adsorption at higher temperatures. The positive value of change in

enthalpy (∆Hº) suggested that Au(III) adsorption was an endothermic process. Also, the positive

value of change in entropy (∆Sº) suggested that there was increased randomness at the solid-

solution interface during the adsorption of Au(III) in aqueous solution on CLP gel.

105

Table 4.2 Thermodynamic parameters for Au (III) by cross-linked paper gel

T(K) b Qmax ∆Gº ∆Hº ∆Sº R2

(dm3 mmol

-1) (mmol g

-1) ( kJ mol

-1) (kJ mol

-1) (kJ mol

-1 K

-1)

298 11.05 4.52 -5.95 0.99

303 9.42 5.05 -5.65 78.09 0.27 0.99

313 27.0 7.40 -8.57 0.99

323 112.0 8.92 -12.67 0.99

q/m

mo

lg

-1

Ce/mmol dm-3

106

Ceq

-1/g

dm

-3

Ce/mmol dm-3

Figure 4.4 Adsorption isotherms of Au(III) on cross-linked paper gel at different temperature (a)

experimental plot, (b) Langmuir plot and (c) van’t Hoff plot. Conditions: dry weight of the gel =


-3

y = -9.3938x + 33.571

R² = 0.9085

0

1

2

3

4

5

3.0 3.1 3.2 3.3 3.4

ln b

1/T × 1000 [K-1]

(c)

107

The comparative accounts for maximum Au(III) adsorption capacity on various

adsorbents listed in Table 4.3 shows the highest adsorption capacity of Au(III) onto CP gel

among the collected literature values. This account also supported the easy recovery and high

efficiency of adsorption for Au(III) using CLP gel.

Table 4.3 Comparison of maximum adsorption capacities for Au(III) with reported

adsorbents together with CLP gel investigated in this study

Adsorbents Qmax Adsorption media References

(mmol g-1

)

Cross-linked paper gel 5.05 0.1 M HCl Present work

Dimethyamine-paper 4.6 1 M HCl [8]


IDA type of modified paper gel 3.30 1 M HCl [10]

Alginate cross-linked with CaCl2 1.47 pH 2 [12]

Bayberry tannin-immobilised silica 2.20 pH 2 [13]

Glutaraldehyde cross-linked chitosan 2.14 pH 1.6 [14]


Cross-linked lignophenol 1.92 0.5 M HCl [16]

Primary amine-cross-linked lignophenol 1.98 0.5 M HCl [17]

Ethylene diamine cross-linked lignophenol 3.08 0.5 M HCl [17]

4.3.5 Solid-State Analysis of Gel After the Adsorption of Au(III)

From the visual observation, it was found that at low concentrations of Au(III) i.e. 0.5

mmol dm-3

, fine gold particles were observed to float on the surface of the sample solution after

24 h of shaking time. On the other hand, after long time shaking (96 h) in the case of high

concentrations of Au(III), these fine gold particles were found to form aggregates, resulting in

the formation of large gold particles as shown in Figure 4.5. The high selectivity and loading

108

capacity of CP gel for Au(III) may be attributed to the reduction of Au(III) to Au(0) during

adsorption. Finally the gel surface was occupied by the reduced gold particles. It was inferred

that fine gold particles formed by the reduction process on the gel surface may have a tendency

to be detached from the gel surface, forming clear aggregates. Thus, the optical microscope of

gel after the adsorption of Au(III) provided a convenient and reliable visual indication of gold

particles formation. Similar phenomena were observed in our previous study in the adsorption of

Au(III) on the chemically modified cotton gel as described in Chapter 3.

Figure 4.5 Optical microscope photograph of gold-loaded cross-linked paper gel

The presence of gold as Au(0) on CP gel was confirmed by XRD analysis. X-ray

diffraction patterns as shown in Figure 4.6 were compared to the reference data base for metallic

gold. No additional peaks, except those attributable to elemental gold i.e. Au(0), were detected

proving that CP gel has the ability to reduce Au(III) ions to metallic gold i.e. Au(0) crystals

structure. The presence of well-defined sharp peaks at 2θ values of 38.18, 44.28, 64.50, and

109

77.40 degree was indicative of well-defined crystals of elemental gold. Similar results had also

been reported for persimmon peel gel.

Figure 4.6 X-ray diffraction powder patterns of Au(III)-loaded CLP showing the presence of

Au(0)

The cross-linked gel before and after the treatment of Au(III) was further analyzed using

scanning electron microscopic (SEM) to evaluate the surface morphology and presence or

absence of newly formed gold particle. The SEM image of CLP before and after Au(III)

adsorption is presented in Figures 4.7 (a) and (b). In the both cases, the surface of the adsorbent

was non porous or smooth, whereas new particles of gold were observed on the surface of CLP

in the case of CLP after Au(III) adsorption which were considered to be zero-valent gold formed

after the adsorptive-reduction of Au(III) ion. These results also supported results of the formation

110

of gold particles after the treatment with CLP gel as elucidated from XRD spectra and digital

microscopic image.

Figure 4.7 SEM micrographs of cross linked paper gel (a) before and (b) after the adsorption of

Au(III). 290 x magnification, acceleration voltage = 20 kV, scale = 30 µm

.

4.3.6 Fourier Transformed Infrared Spectra

Paper is one of the cellulose rich bio-polymers. Since hydroxyl groups are very abundant

in polysaccharides, like cellulose, their participation in the adsorption followed by reduction

process in the present system was confirmed by FT-IR analysis of CLP gel. The characteristic

intermolecular and intra-molecular O-H stretching vibration band of hydroxyl groups in the

spectrum of crude paper was observed and displayed in Figure 4.8 (a), which was slightly less

broad, showing the maximum intensity at around 3446 cm-1

. The corresponding band appeared

in the spectrum of CLP gel as presented in Figure 4.8 (b) at around 3417 cm-1

, which was

(b)(a)

111

broader compared to the crude paper band. This suggested that some of the OH groups in crude

paper cellulose were consumed in the cross-linking condensation reaction. The intense peak at

2913 cm-1

in crude paper was derived from C-H stretching, the sharp peak at around 1716 cm-1

was assigned to the C=O stretching vibration of carbonyl groups of paper cellulose, and a broad

band centered at 1176 cm-1

was attributed to the C-O stretching and O-H bending of alcoholic

groups. Furthermore, a new peak was appeared in CLP gel at around 1215 cm-1

which was

identified by the formation of C-O-C ether bond linkages, indicating strong evidence of the

cross-linking condensation reaction by concentrated sulphuric acid. After the adsorption of

Au(III) ion, the broad band at 3417 cm-1

has become broader, which was attributable to the

coordination of adsorbed Au(III) ion with oxygen atom of hydroxyl group because metal

coordination weakened O-H bond and absorption shifted towards the lower frequency region as

depicted in Figure 4.8 (c). Similarly, the intensity of peak at 1716 cm-1

increased after Au(III)

adsorption. It was reasonably explained to be attributable to the oxidation of the hydroxyl groups

to carbonyl group in CLP gel which confirms the reduction of ionic gold to elemental gold.

112

Figure 4.8 FT-IR spectra of (a) crude, (b) cross-linked and (c) Au-loaded paper gels

(b)

400100016002200280034004000

Wave number (cm-1)

(c)

(a)

Tra

nsm

itta

nce

%

113

4.3.7 Recovery of Gold from Au-Loaded Adsorbent by Incineration

Very beautiful, shining, and twinkling gold aggregates attached on the surface of

adsorbent were observed by optical microscope as shown in Figure 4.5. Such gold was easily

recovered in its pure form by incineration. The thermo gravimetric curve of CLP and Au-loaded

CLP were recorded and presented in Figure 4.9, which shows the TG curves of the CLP gel

loaded and unloaded with gold, the variation of the percentage of remained weight of the gels as

a function of elevated temperature. In the case of unloaded CLP (Figure 4.9 a), the gel particles

were nearly vanished after the treatment of heat at high temperature. In the case of TG curve of

Au-loaded CLP, the gel was completely decomposed at temperature higher than 500°C leaving

the weight of clear and pure gold aggregates as shown in Figure 4.9 (b) indicating that gold was

easily recovered with high purity from the Au-loaded CLP via incineration at high temperature.

The weight loss at the temperature below 150°C was caused by the evaporation of the adsorbed

H2O molecules in both crude and gold loaded gel.

The weight loss in the temperature range from 150 to 500°C (gold loaded) and in that

from 150 to 520°C (unloaded gel) was attributed to the decomposition of organic molecules by

incineration reaction releasing H2O and CO2. It was found that, 30% of the initial weight

remained as very beautiful, shining, and twinkling gold after the incineration of the Au-loaded

CLP (Figure 4.9 c) at 1000ºC while the percentage of weight remained in unloaded gel was very

small (0.75%). Practically, the incineration process is useful for recovering pure metallic gold in

various industrial fields. Thus, it was expected that the CLP gel investigated in this work is a

promising material to recover elemental gold from industrial effluents of gold plating and mining

industries that contains trace amount of gold ion.

114

Figure 4.9 Thermo-gravimetric analysis of cross-linked paper gel (a) before gold loading, (b)

after gold loading and (c) recovered gold after incineration

4.3.8 Proposed Mechanism for Adsorption Followed by Reduction of Au(III)

Several spectroscopic methods have been used to elucidate the mechanisms of gold

adsorption and reduction processes. Most of the mechanism involved the reduction of ionic gold

to metallic form. On the basis of FT-IR analysis, Torres et al., suggested the involvement of

carboxyl groups in binding followed by the reduction of Au(III) to Au(0) and Ag(I) to Ag(0), in

the case of metal binding onto cross-linked calcium alginate beads (Torres et al., 2005).

The main adsorption-reduction mechanism of Au(III) in the present system was

investigated based on the structural changes observed in FT-IR spectra for raw paper and CP gel.

The reduction of ionic gold to elemental gold was proved by measuring the solid state analysis of

the gel after adsorption by XRD, SEM, and optical microscope images. The polymeric structure

115

of paper cellulose, in the presence of concentrated sulphuric acid, underwent the condensation

cross-linking reaction forming C-O-C ether bonds linkage (as observed in the FT-IR studies)

with the release of water molecules as shown in Scheme 4.1. The majority of Au(III) ions in

acidic chloride media exists predominantly as a tetra-chloro complex of AuCl4-. The adsorption

of this complex took place through the coordination with oxygen atom of C-O-C linkage of ether

bond and oxygen atoms of many other hydroxyl groups of pyranose ring of paper cellulose as

shown in Figure 4.10. At this step of the adsorption, one chloride ion was released per unit ion

of Au(III) into the aqueous solution, which resulted in the decrease in the adsorption of gold as

observed in high concentration range of HCl in Figure 4.2. Furthermore, during the adsorption,

some hydroxyl groups were oxidized to carbonyl groups leading to the reduction of Au(III) to

Au(0) as discussed, concerning the changes in FT-IR spectra. Thus, the very high adsorption

capacity and selectivity of paper gel for Au(III) was attributable to the change in the structure of

the polymer matrices of paper cellulose by a simple cross-linking reaction using concentrated

sulphuric acid. Another driving force for the reduction of Au(III) to Au(0) is the high oxidation-

reduction potential (ORP) of Au3+

(1.0 V) in comparison to other metal ions like Pd2+

(0.68 V)

and Pt4+

(0.62 V), which bring about selective reduction for Au(III). Thus, it was inferred that the

mechanism of the adsorption of Au(III) on CP gel followed by the reduction to metallic gold is

schematically depicted by Figure 4.10.

116

O

H

H

H

O

H

H

H

OO

O Au

OH

O

H

Cl

Cl

Cl

H

O

n

m

O

H

H

H

O

H

HH

OO

O

O

H

H

O

n

m

O

O

CH2OH

CH2OH

CH2OH

CH2OH

H

O

H

H

H

O

H

H

H

H

OO

O

O

H

H

O

n

m

CH2OH

CH2OH

OH

OH

4H++ 3Cl-+ Au0

Cross- linked paper gelAu(III) adsorbed paper gel

Formation of elemental gold on paper gel

Au(

III)

redu

ctio

n

+

AuCl4-

+

Cl-

OHH

Adsorption

H

Figure 4.10 Inferred adsorption-reduction mechanism for Au(III) using cross-linked paper gel

117

4.4 CONCLUSIONS

The applications of the paper cellulose for the recovery of gold have been investigated in

terms of adsorption tests, spectroscopic analyses, and visual observations. The values of

thermodynamic parameters confirmed the spontaneous and endothermic reaction. Kinetic data

was successfully interpreted in terms of pseudo-first order model with the activation energy of

61.3 kJ mol-1

. Thus, high adsorption capacity and selectivity of the present gel towards Au(III)

offered an ecological and economic alternative to the procedures currently used to treat mining

and industrial effluents. The present work suggested the possibility of very simple and new uses

of cellulosic material, like paper, as low cost adsorbent for the effective recovery of gold from

various wastes, especially from e-wastes in its pure form.

118

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Hazard. Mater., 158, 228-256.

2. Ishikawa SI, Suyama K, Arihara K, Itoh M, (2002) Uptake and recovery of gold ions from

electroplating wastes using eggshell membrane, Bioresour. Technol., 81, 201-206.

3. Li J, Lu H, Guo J, Xu Z, Zhou Y, (2007) Recycle technology for recovering resources and

products from waste printed circuit boards, Environ. Sci. Technol., 41, 1995-2000.

4. Akita S, Yang L, Takeuchi H, (1996) Solvent extraction of gold(III) from hydrochloric acid

media by nonionic surfactants, Hydrometallurgy, 43, 37-46.

5. Alguacil FJ, Adeva P, Alonso M, (2005) Processing of residual gold(III) solutions via ion

exchange, Gold Bulletin, 38, 9-13.

6. ZhaoYZ, (2006) the enrichment and separation of race gold, Pt and Pd from the ores based

on co-precipitation, Gold, 27, 42-44.

7. Qu R, Sun C, Wang M, Ji C, Xu Q, Zhang Y, Wang C, Chen H, Yin P, (2009)

Adsorption of Au(III) from aqueous solution using cotton fiber/chitosan composite

adsorbents, Hydrometallurgy, 100, 65-71.

8. Adhikari CR, Parajuli D, Kawakita H, Inoue K, Ohto K, Harada H, (2008 a)

Dimethylamine modified waste paper for the recovery of precious metals, Environ. Sci.

Technol., 42, 5486-5491.

9. Adhikari CR, Parajuli D, Inoue K, Ohto K, Kawakita H, Harada H, (2008 b) Recovery of

precious metals by using chemically modified waste paper, New J. Chem., 32, 1634-1641.

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10. Adhikari CR, Parajuli D, Kawakita H, Chand R, Inoue K, Ohto K, (2007) Recovery and

Separation of precious metals using waste paper, Chem. Letters, 36, 1254-1255.

11. Parajuli D, Adhikari CR, Kawakita H, Kajiyama K, Ohto K, Inoue K, (2008) Reduction and

accumulation of Au(III) by grape waste: A kinetic approach, React. Funct. Polym, 68,

1194-1199.

12. Torres E, Mata YN, Blázquez ML, Muñoz JA, González F, Ballester A, (2005) Gold and

silver uptake and nanoprecipitation on calcium alginate beads, Langmuir, 21, 7951-7958.

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120

CHAPTER 5CHAPTER 5CHAPTER 5CHAPTER 5

Adsorptive Recovery of Trace Concentration of Au(I) from Model Solution in

Sodium Hypochlorite Media

Although we have succeeded to recover the trace concentration of Au(III) from hydrochloric acid

media by using cross-linked polysaccharide gels, our final target is to apply our technology in industrial

processing. The sulphite and cyanide salts of Au(I) are the most common chemicals used in gold plating

industries where large amount of waste solution containing trace amount of Au(I) was exhausted.

Moreover, for the extraction of gold from its ore and from the various electronic appliances, cyanide

leaching is most effective and common. The gold plating waste solution and cyanide leached liquor of

gold ore contains gold in the forms of Au(CN)2- or Au(SO3)2

-, thus recovery of gold from such solutions

is required. The gold cyanide is very toxic compared to gold sulphite solution. It was found from the

literature that Au(CN)2- or Au(SO3)2

- possesses chemical similarity thus forms similar types of complex in

aqueous solution. In the present chapter, the new way of recovering trace concentration of Au(I) from less

toxic Au(SO3)2- solution has been investigated choosing it as a model solution of Au(CN)2

-.

5.1 INTRODUCTION

Gold is one of the most important noble metals due to its wide applications in industry

and economic activity, yet it is not naturally abundant1. Gold is commonly found associated with

each other and have important applications, but a significant partition of their demand is for

jewelry, coinage and decorative arts. Usually mined ore is ground to expose the gold particles

and then leached using cyanide. The cyanide solution is most widely use for gold mining and

121

plating applications because of its special complexing capabilities in aqueous solution. This

creates a soluble Au(CN)2- complex, which is further processed to recover the gold. Various

effluents generated from such process contain significant amount of Au(CN)2- complexes. Such a

cyanide complexes of Au(I) was conventionally treated with strong oxidizing agent like H2O2,

NaClO etc. in alkaline medium for the cyanide decomposition which is then subjected to

recovery step using chemical reduction, ion exchange or adsorption onto activated carbon

described in detailed later. However, cyanide is highly toxic and the environmental conditions

governing its use in the gold industry are becoming increasingly stringent. The debate

surrounding the use of cyanide in the mining industry has fuelled considerable investigation into

the development of more environmentally benign alternatives2. However, there are several

shortcomings to the use of gold cyanide which have stimulated the investigation and

commercialization of other gold complexes in plating processing. The stability of the gold

cyanide complex causes the reduction potential to occur at very negative potentials, resulting in

the co-reduction of hydrogen ions, which lowers the plating efficiency and makes the

developments of electro less plating processing difficult. The release of free cyanide during the

reduction of Au(CN)2- is also the incompatible. Lastly, the health and safety of workers and the

environmental impact of the wide-scale use of cyanide are a concern.

Generally, trace concentration of precious metals remaining in wastewaters generated

from the refining process of precious metals is difficult to be recovered due to a relatively high

processing cost as well as various technical problems. Recovery of precious metals from

wastewaters is very important for the conservation of resource and the protection of environment.

The method such as ion exchange is also popular and used for the recovery of gold3-5

.

Conventionally, Au(I) was recovered from cyanide solution by using activated carbon and

122

cementation using zinc powder6. The use of activated carbon for the treatment of leached

solution containing dissolved gold, involves the process called carbon-in-pulp (CIP). The process

provided an alternative to the gold recovery processes using filtration or counter-current

decantation in conjunction with the Merrill-Crown zinc precipitation process7. The serious

drawback of using activated carbon and ion exchange resins as the adsorbents was the difficulty

for the recovery of trace concentration of gold from the effluent solution because the finely

crushed gold ore after leaching with sodium cyanide solution entered into the pore of activated

carbon and ion exchange resin at the time of adsorption process. The pores are tightly packed

and some trace amount of gold will remain inside the pores. So, these adsorption processes have

some disadvantages such as difficult to absorb the trace concentration of gold due to clogging

onto the pores and regeneration is also difficult.

Recently, biosorption research has revealed the strong potential for precious metal

recovery because of their easiness to regenerate and reuse8, 9

. The loaded gold onto these

adsorbents can recover into its elemental form simply by incineration process at high

temperature10

. In our previous work assessing the recovery of trace concentration of Au(III) from

aqueous medium as mentioned in our published paper, the cross-linked polysaccharide

successfully recovered Au(III) from trace concentration even in the presence of other precious

and base metals which usually coexisted in the system in its zero-valent gold. It was observed

that various polysaccharide-based adsorbents after cross-linking with concentrated sulphuric acid

was excellent material for adsorbing the Au(III) and subsequently reduced to its elemental form11

.

Based on these results, we further expected to attempt the quantitative recovery of gold(I) from

hypochlorite media using gold sulphite. Here the use of sodium hypochlorite solution as a strong

oxidizing agent (which can decompose cyanide) was for the oxidation of Au(I) into Au(III).

123

Thus, the Au(III) was adsorbed and consequently reduced into elemental gold by the use of

cross-linked cellulose gel treated with concentrated sulphuric acid. In the industries,

detoxification of cyanide solution was performed by using strong oxidizing agent such as NaClO

at basic condition by Eq. (5.1)12

.

2 NaCN + 5 NaClO + H2O = 2 NaHCO3 + N2 + 5 NaCl (5.1)

The cyanide solution used in leaching of gold ore and gold cyanide solution applied in

the gold plating industries are very much toxic so that uses of such toxic chemicals are limited.

Another alternative insignificantly toxic chemicals also widely applied in gold plating factories is

gold sulphite which possesses similar chemical behavior with gold cyanide and forms the similar

types of complex in aqueous solution. Thus, we used sodium salt of Au(I) sulphite i.e.

Na3(Au(I)SO3)2 for adsorptive recovery of Au(I) alternative to cyanide solution. The basic work

for recovering gold(I) from spent cyanide gold plating solution was conducted by using the gold

sulfite solution in laboratory scale. The use of gold sulfite complex for gold plating was known

since 184213

and it is still the most commonly used to prepare the gold complex in non-cyanide

process. Gold sulphite processing traditionally attracted the attention because of their ability to

produce bright, smooth and ductile pure gold deposited than that with cyanide processing. In the

present investigation, the Au(I) from gold sulphite solution was first oxidized into Au(III) by the

use of strong oxidizing agent NaClO by the aid of sufficient amount of HCl solution to

successfully recover as a elemental gold in two step process consisting of adsorption onto cross-

linked cellulose followed by incineration. The influence of different parameters on the

adsorption of trace concentration of Au(I) species from the aqueous gold sulphite (alternative to

spent cyanide gold plating solution) in hypochlorite media onto the solid adsorbent has been

formulated.

124


5.2.1 Preparation of Concentrated H2SO4 Cross-linked Pure Cellulose Gel

Preparation of cross-linked pure cellulose (CLPC) gel for the recovery of gold was described

in Chapter 2 in details which was briefly discussed as follows. For this purpose, 10 g of pure

cellulose (Crystalline cellulose powder (102330 Cellulose) marketed by Merck, Germany, for

thin layer chromatography) was heated with 50 cm3 of concentrated H2SO4 for 24 h at 100ºC for

condensation cross-linking reaction. After that, the mixture solution was cooled at room

temperature and was neutralized with NaHCO3 and washed several time with distilled water until

the pH of the washing water reach around pH 7. Thus, obtained black product was dried in a

convection oven for 24 h at 70ºC. Then, the gel was crushed by mortar and pestle. The gel was

sieved by testing sieve in order to regulate the particles size (<75µm) which was utilized for the

recovery of Au(I) from Au(SO3)2- solution.

5.2.2 Chemicals and Material Used

The stock solution (10 mmol dm-3

) of Au(I) solution was prepared by dissolving 5 cm3

of sodium gold-sulfite (Na3(Au(I)SO3)2 solution purchased from Tanaka Kikinzoku Kogyo, K.K.,

Tokyo, Japan containing 100.2 g dm-3

of gold in 250 cm3 of deionized water. The working

solutions were freshly prepared from the stock solution at the time of experiment by dilution. In

order to oxidize the Au(I) to Au(III), sodium hypochlorite solution (NaClO) was used (Wako

Pure Chemical Industries, Ltd,. Japan). Adjustment of pH was carried by 0.1 mol dm-3

HCl and

0.1 mol dm-3

NaOH. The metal solutions of Pt(IV), Pd(II), Fe(III), Zn(II) and Cu(II) were

prepared from respective chloride salts as mentioned in the earlier chapters. All other chemicals

125

used for the preparation of the adsorbent and for the subsequent adsorption tests were of

analytical grade and were used without further purification.

5.2.3 Batch-wise Studies of Au Adsorption in NaClO Media

The adsorption of Au(I) was carried out by means of batch-wise type of method. The

adsorption of Au(I) was carried out by shaking 10 mg of CLPC gel together with 10 cm3 of the

gold solutions at different conditions for 30 h at a shaking speed of 150 rpm and temperature of

30ºC to ensure complete equilibrium. After shaking, the equilibrated solution was filtered and

analyzed for the gold concentrations using either ICP-AES or AAS. The percentage adsorption

of the metal ions (% A) and metal uptake capacity q, mmol g-1

were calculated by Eqs. (5.2) and

(5.3).

% A 100×−

=

i

ei

C

CC (5.2)

VM

CCq ei

×

−

= (5.3)

where Ci and Ce are the initial and equilibrium concentrations of metal ions (mmol dm-3

),

respectively, V is the volume of the metal solution (dm3) and M is dry mass of the adsorbent (g).

Kinetic measurement was performed by changing contact time between the adsorbents and

metal solutions at a solid/liquid ratio of 1 g dm-3

. The adsorption isotherm test for Au(I) on

CLPC gel was carried out by varying the Au(I) concentration (0.5 - 10 mmol dm-3

). With the

exception of the kinetics tests, the mixtures used for the adsorption isotherm tests were stirred for

30 h at 30ºC. The metal ion concentrations in the tested samples before and after adsorption were

then measured by ICP-AES or AAS.

126


5.3.1 Elemental Analysis of CLPC Gel Before and After Adsorption of Au by EDX

The direct evidence of Au(I) adsorption onto cellulose gel was confirmed by energy

dispersive X-ray (EDX) spectroscopy. The EDX spectra of the CLPC gel taken before and after

Au(I) adsorption are shown in Figure 5.1. As seen from Figure 5.1 (a), the peaks corresponding

to C, O, Na, Si, P, S, K, Ca, Mn, and Fe were observed for cellulose gel before Au(I) adsorption

at energy values 0.27, 0.56, 1.01, 1.51, ,1.75,2.13 and 2.32, 2.74,3.76, 5.44, 6.42 and 7.12 keV.

After Au(I) adsorption (in NaClO media with hydrochloric acid) as shown in Figure 5.1(b), new

peaks related to gold were observed at energy values of 8.4, 9.74 and 11.56 keV in addition to

the peaks mention in CLPC in Figure 5.1(a), suggesting that the Au(I) had been certainly

adsorbed onto CLPC gel. Thus, this results provided some indication of the strong affinity of the

present adsorbent for Au(I) from acidic NaClO media.

0.0005

0.005

0.05

0.5

0 2 4 6 8 10 12

Inte

nsit

y

keV

OC

NaSi

P

S

K

Ca

Mn

Fe

Fe

(a)

127

Figure 5.1 Energy dispersive X-ray spectra of (a) cross-linked cellulose gel, and (b) cross-linked

cellulose gel after Au-adsorbed

5.3.2 Recovery of Au(I) without NaClO and HCl Solution

In order to know whether the trace concentration of Au(I) without the use of NaClO and

HCl is recovered by using cellulose gel or not, adsorbent dose test for Au(I) recovery was

conducted as shown in Figure 5.2. However, there was negligible adsorption of Au(I) in the

absence of NaClO and HCl. This result also became supportive for our assumption of Au(I)

recovery, that is, it suggested that the strong oxidizing agent NaClO helps for the oxidation of

Au(I) to Au(III) and that of HCl was responsible for the formation of chloro-complex of Au(III)

as described in the earlier section which was effectively adsorbed on the surface of CLPC gel.

0.0005

0.005

0.05

0.5

0 2 4 6 8 10 12

Inte

nsit

y

keV

C

O

Na

S

K

Ca

P

S

Mn

Fe

Fe

Au

Au

Au

Si

(b)

128

Figure 5.2 Effect of solid/liquid ratio for the recovery of Au(I) from Na3(Au(I)SO3)2 solution

prepared in water without using NaClO and HCl. Condition: [Au(I)] = 0.3 mmol dm-3

, volume of

solution = 10 cm3, temperature = 303 K, shaking time = 30 h, shaking speed = 150 rpm (Initial

pH = 7.56 itself)

5.3.3 Effect of NaClO Concentration and Adsorption Mechanism of Au(I) After Oxidation

Figure 5.3 depicts the percentage adsorption of Au(I) as well as other precious and base

metals as a function of NaClO concentration. The result clearly showed that over the whole

NaClO concentration from 0.01 mol dm-3

to 1 mol dm-3

, the adsorption of gold was 100% i.e.

quantitative adsorption was achieved even at 1 mol dm-3

NaClO concentration using 0.1 mol dm-

3 HCl concentration. On the contrary, there was negligible adsorption of other precious and base

metals tested in the experiment. The result provided the strong selectivity of this adsorbent for

Au(I) ion from the mixed solution compared to other precious and base metals indicating that

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12

Rem

ain

ing

Au

(I)

co

nce

ntr

ati

on

[m

g d

m-3

]

Solid/liquid ratio [g dm-3]

129

selective recovery of Au(I) was successfully achieved from Au(SO3)2- solution from NaClO

medium in the presence of 0.1 mol dm-3

HCl. Thus it was expected that the investigated

adsorbent can be used to recover the Au(I) from Au(CN)2- due to the chemical similarity with

Au(SO3)2- solution.

Figure 5.3 Effect of NaClO concentrations for the adsorption of metal ions. Conditions: metal

concentrations = 0.2 mmol dm-3

, [HCl] = 0.1 mol dm-3

, shaking time = 24 h, shaking speed = 150

rpm at 303 K, volume of solution = 10 cm3, amount of gel = 10 mg

It was assumed that Au(I) was oxidized to Au(III) with the aid of NaClO and adsorbed onto

the cellulose gel as described in the earlier chapters. Initially Au(SO3)2- i.e Au(I) solution is

colorless which after the addition of excess amount of NaClO in the presence of HCl was converted

into pale yellow color similar to the color of AuCl4- i.e. Au(III) solution. In order to further

elucidate the conversion of Au(I) to Au(III), UV-visible spectra of Au(III) solution and Au(I)

solution after the treatment with NaClO in the presence of HCl was recorded together with Au(I)

solution as presented in Figure 5.4.

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

% A

dso

rpti

on

of

me

tals

NaClO (mol/dm3)

Au(I)

Pt (IV)

Pd (II)

Fe (III)

Cu (II)

130

Figure 5.4 UV-visible spectra of Au(I), Au(III) and NaClO treated Au(I) in the presence of HCl

showing the oxidation of Au(I) to Au(III)

It showed that colorless solution of Au(I) did not give the peaks at around 250 to 350 nm

whereas in the case of Au(III) solution and NaClO treated Au(I) solution, strong absorption peaks

was observed in the range of this wavelength. Thus Au(I) solution was concluded to be converted

into Au(III) according to the Eqs.(5.4) and (5.5). The AuCl4– solution formed in this way was

effectively adsorbed onto the CLPC gel as described earlier in Chapter 2 via coordination with

hydroxyl ions of cellulose and C-O-C linkage formed after cross-linking of cellulose in CLPC gel.

NaClO + HCl � HOCl + NaCl (5.4)

2Au(I) + 3HOCl + 3H+ + 5Cl ̄ �2[AuCl4] ̄ + 3H2O14

(5.5)

5.3.4 Kinetic Experiment

Kinetic measurement is necessary for determining the optimal contact time required to

reach equilibrium. The relationship between the amount of Au(I) adsorption onto the cellulose

0

0.1

0.2

0.3

0.4

250 300 350 400

Ab

sorb

an

ce

Wave length /nm

Au(III) initial 5 ppm

Au(I) initial 5 ppm

Au(I) 5ppm in 1M NaClO

131

gel and contact time is shown in Figure 5.5 Adsorption of Au(I) onto the CLPC gel increases

sharply at the beginning of contact and then gradually slowed down, finally reaching equilibrium

within 26 h. Consequently, the solid-liquid mixtures were shaken for 30 h to ensure complete

equilibrium in subsequent experiments.

Figure 5.5 Effect of contact time for the adsorption of Au(I) onto cross-linked cellulose gel.

Conditions: concentration of Au(I) = 0.5 mmol dm-3

, volume of the solution = 250 cm3, weight

of the gel = 250 mg, [NaClO] = 1 mol dm-3

, [HCl] = 0.1 mol dm-3

, temperature = 303 K

5.3.5 Effect of Adsorbent Dose

Figure 5.6 shows the remaining Au(I) concentration in the treated solution as a function

of solid/liquid ratio. The result clearly demonstrated that the remaining concentration of Au(I)

was drastically decreased with the increase of adsorbent dose which was attributed to the fact

that the number of active adsorption sites for Au(I) was increased with the increase in adsorbent

amount that consequently lead to the removal of Au(I) from the solution. The result also

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40

Am

ou

nt

up

tak

e o

f A

u(I

) [

mm

ol/

g]

Time [h]

132

indicated that on increasing the adsorbent dose from 0.5 to 1 g dm-3

, the remaining concentration

of solution Au(I) was found to be negligible in the solution, suggesting that Au(I) was effectively

adsorbed from Na3(Au(I)SO3)2 solution. Thus, the observed experimental data suggested that

using small amount of solid/liquid ratio, the quantitative recovery of gold (I) was achieved and it

was expected that Na[Au(CN)2] was also recovered using NaClO in the presence of HCl.

Figure 5.6 Effect of adsorbent dose for the recovery of gold using cross-linked cellulose gel.

Condition: Au(I) = 0.3 mmol dm-3

prepared in water, [NaClO] = 0.1 mol dm-3

, pH = 3,

temperature = 303 K, shaking time = 24h, volume of solution = 10 cm3, shaking = 150 rpm.

The effect of pH on the Au-adsorption test from 1 mol dm-3

NaClO containing 0.5 mmol

dm-3

Au(I) was presented in Figure 5.7. It showed that the Au(I) adsorption was maximum at pH

between 3-6 and decreased pH higher than 6. In order to find out the effective ORP values for

the recovery of Au(I), the oxidation-reduction potential (ORP) values at different pH was

recorded and also presented in this figure. It showed that ORP values between 750-1200 mV was

found to be effective for Au(I) recovery by using CLPC gel.

0

10

20

30

40

50

60

70

0 2 4 6 8 10

rem

ain

ing

Au

(I)

con

cen

trati

on

[m

g d

m-3

]

Solid/liquid ratio [g dm-3]

133

Figure 5.7 Adsorption test of Au(I) as a function of pH. Condition: Initial [Au (I)] = 0.5 mmol

dm-3

, [NaClO] = 1 mol dm-3

, Shaking time = 24 h, shaking speed = 150 rpm at 303 K

5.3.6 Recovery Attempt of Au(I) from Aqua Regia Solution

In the preceding section, Au(I) was oxidized into Au(III) using NaClO, in relation to which

it was attempted to oxidize using aqua regia (3:1, conc. HCl: conc.HNO3), a typical oxidizing

lixiviant for gold. That is, recovery of trace concentration of Au(I) was attempted from aqua regia

solution. For this purpose, definite amount of Au(I) was added in aqua regia and the mixture was

shaken on water bath at 40°C for 11 days to supply for the adsorption test of Au(I), which was

conducted by shaking different amount of the adsorbent with 10 cm3 of this solution for 30 h. After

filtration, the concentration of Au(I) was measured by ICP-AES.

However, it was found that no Au(I) was adsorbed onto CLPC gel because, in this case, the

adsorbent, CLPC, was completely decomposed by aqua regia solution resulting the destruction of

polymer matrix of cellulose or active sites leading to negligible adsorption of Au(I). The destruction

of CLPC gel was also observed visually. After the treatment of CLPC with Au(I) containing aqua-

regia solution, black color solution was observed and size of the adsorbent particles are decreased

0

200

400

600

800

1000

1200

1400

0

20

40

60

80

100

1 2 3 4 5 6 7 8 9 10

OR

P m

V

% A

dso

rpti

on

of

Au

pHe

134

which is due to the dissolution of CLPC gel in aqua regia solution. From this result, it was

concluded that recovery of Au(I) from aqua regia medium is difficult using CLPC gel which is one

of the limitation of our adsorbent. To overcome such problem, we have adjusted the pH of the aqua

regia solution by adding NaOH to pH 4 and the adsorption test of Au(I) was carried out from such

solution. The result is shown in Figure 5.8. The residual concentration of Au(I) after adsorption was

found to decrease from 55.6 to 11.5 mg dm-3

by using 0.05 g dm-3

of CLPC whereas it was lowered

at around 0.14 mg dm-3

at 1 g dm-3

. In order to successfully achieved the 100% removal of Au(I),

higher than 2 g dm-3

of CLPC was required. The decreased in residual concentration of Au(I) with

the increase of adsorbent dosage was reasonably attributable to the fact that the number of active

sites per unit volume of adsorbent increased with increasing adsorbent dosage that ultimately

increased the Au(I) removal.

Figure 5.8 Adsorption test of Au(I) from aqua regia after adjusting pH to 4 by adding enough

amount of NaOH. Condition: Initial [Au(I)] = 0.5 mmol dm-3

, volume of solution = 10 cm3, Initial

pH = 4, shaking time = 24 h, shaking speed = 150 rpm, ORP value after adjusting pH to 4 = 676.8

mV

0

10

20

30

40

50

60

0 5 10 15

Au

re

ma

ing

co

nce

ntr

ati

on

mg

dm

-3

solid/liquid ratio g dm-3

135

5.3.7 Adsorption Isotherm Test of Au(I) After Oxidation at Different Temperature

Figure 5.9 (a) shows the adsorption isotherms of Au(I) onto CLPC gel from 1 mol dm-3

NaClO solution at varying temperature. The result showed that adsorption of Au(I) increased with

increasing concentration of Au(I) ions and reached saturation at higher concentration indicating the

Langmuir type monolayer adsorption on the surface of adsorbents. For the evaluation of maximum

uptake capacity of Au(I) ion by cellulose gel, the experimental data were modeled by using well

known Langmuir isotherm model. Langmuir isotherm can be represented by Eq. (5.6).

maxmax

1

q

C

bqq

C e

e

e+= …………………………………………… (5.6)

where Ce (mmol dm-3

) and qe (mmol g-1

) are equilibrium concentration and amount of adsorption,

respectively. The qmax is the maximum loading capacity and b is the adsorption equilibrium constant

or binding constant related to the adsorption energy. The result showed that experimental data was

in good agreement with the Langmuir isotherm with high correlation coefficient i.e r2= 0.999 in

each temperature studied. The values of qmax and b were evaluated from the slope and intercept of

the straight lines for the Ce/qe versus Ce plot {Figure 5.9 (b)}, respectively. Thus calculated

Langmuir constant or equilibrium constant (b) was used for the determination of thermodynamic

parameters at three different temperature studied. The thermodynamics parameters such as change

in Gibbs free energy (∆G°), change in enthalpy (∆H°) and change in entropy (∆S°) were calculated

using Eqs.(5.7) and (5.8),

∆G° = -RTln b (5.7)

∆G° = ∆H°- T∆S° (5.8)

By merging Eqs. (5.7) and (5.8), the van’t Hoff equation is expressed as follows:

136

lnb = ∆B°

C − ∆D°

CE (5.9)

As presented in Figure 5.9 c, the plot of lnb versus 1/T gives a straight line with a slope of (−∆D°

C ) kJ

mol-1

and an intercept of �∆B°

F ) kJ mol-1

K-1

. The evaluated values of thermodynamics parameters are

listed in Table 5.1

Table 5.1 Thermodynamic parameters for Au(I) adsorption by cross-linked cellulose gel

T(K) lnb Qmax ∆Gº ∆Hº ∆Sº R2

(mmol g-1

) ( kJ mol-1

) (kJ mol-1

) (kJ mol-1

K-1

)

303 2.66 6.06 -6.71 39.01 0.15 0.99

313 2.99 7.14 -7.79 0.99

323 3.62 8.69 -9.74 0.99

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6

Am

ou

nt

of

Au

(I)

up

tak

e /m

mo

l g

-1

Ce [mmol dm-3]

303 K

313 K

323 K

0

0.4

0.8

1.2

0 1 2 3 4 5 6

Ce

qe-1

/g d

m-3

Ce [mmol dm-3]

(b) (a)

137

Figure 5.9 Adsorption isotherm of Au(I) using CLPC gel. (a) Experimental plot, (b) Langmuir plot,

and (c) van’t Hoff plot. Condition: weight of gel = 10 mg, volume of solution = 10 cm3, [NaClO] =

1 mol dm-3

, [HCl] = 0.1 mol dm-3

, shaking time = 96 h, shaking speed 150 rpm

5.3.7 XRD Measurement of Au -Loaded Cellulose Gel

The formation of metallic gold by the cross-linked cellulose gel was confirmed by XRD

analysis, as shown in Figure 5.10. The XRD pattern for the cellulose gel before the adsorption of

Au(I) was totally amorphous structure, that indicated an unorganized structure of sulphuric acid

treated polysaccharide materials inside this adsorbent. After Au(I) adsorption, the XRD patterns

revealed that intense peaks corresponding to the crystalline structure of Au(0) were clearly

obtained for the cross-linked cellulose adsorbent at the 2θ values of 37.8, 44.0, 64.38 and

77.8.Thus, in the present investigation, it was attempted that the term “adsorption” was used to

describe the phenomenon during which the metal species were loaded into the surface of the

solid adsorbent and then the adsorbed gold was reduced to elemental state which would be

y = -4.6927x + 18.089

R² = 0.9488

1

2

3

4

3 3.1 3.2 3.3 3.4

lnb

T-1 * 1000 [K-1]

(c)

138

further confirmed from the observation of metallic gold in the optical photograph in the later

section.

Figure 5.10 XRD patterns of the cross-linked cellulose gel after contacting with Au(I) solution

5.3.8 Optical Microscope Image of Filtered Cake of Cellulose Gel After Gold Adsorption

The formation of elemental gold was further confirmed from the optical microscope

photographs as shown in Figure 5.11(a), as this gel showed very fine and clear aggregated

particles of elemental gold and were distinctly visible after long period of shaking at high

concentration of Au(I) solution. On the other hand, for short shaking period at low concentration

of Au(I) for example, 0.2 mmol dm-3

, only microscopic gold particles were observed to be

floated on the surface of sample solution. Here, the surface of pure metal was hydrophobic

resulting in the floatation of pure metal particles on water surface. Both the results from XRD

and observation analyses confirmed the effective adsorption of Au(I) on to CLPC gel prepared

0

300

600

900

1200

1500

1800

2100

2400

2700

10 20 30 40 50 60 70 80

Inte

nsi

ty

Degree (2 theta)

37.8

44.0

64.38 77.8

139

from simple chemical modification. Extremely high selectivity and loading capacity of this gel

for Au(I) may be attributed to the reduction to Au(0) during adsorption. It was inferred {from the

results of Figure 5.11(b)} that gold particles formed by the reduction process had a certain sort

of tendency to be detached from the gel surface forming clear aggregates. Similar observations

were also reported in our previous studies on the adsorption of Au(III) using cross-linked paper

and cotton as described in earlier Chapters.

Figure 5.11 Optical microscopic images of elemental gold observed onto the cellulose gel (a)

after Au(I) adsorption, and (b) & (c) during Au(I) adsorption , volume of the solution = 10 cm3,

weight of adsorbent = 10 mg, shaking time (b) = 96 h (2 mmol dm-3

) and for (c) = 24 h (0.2

mmol dm-3

)

5.3.9 NaClO in Hydrochloric Acid

When chlorine is added to water it forms hypochlorous acid (HOCl), a strong, fast-acting

oxidizer. As pH increases, the HOCl disassociates to its ionic form, OCl- (the hypochlorite ion),

which is weaker and slower acting. Consequently, as the pH of a chlorine containing solution

increases, the oxidizing capability decreases and is reflected directly in the ORP reading. An

oxidizer is a chemical that has the ability to accept electrons. In solution, an oxidizer will raise

(a) (b) (c)

140

the ORP value. The greater the oxidizer concentration, the faster will be the rate of reaction.

Thus, the actual ORP depends on both the concentration and the activity of the oxidizer. The

sodium hypochloride employed in this study was converted into HOCl in the presence of

hydrochloric acid according to the reaction expreessed in the reaction Eq. (5.10).

NaClO + HCl → NaCl + HOCl (5.10)

The acitivity of NaClO to oxidize Au(I) to Au(III) was higher at pH lower than 6 because

at this pH range most of the hypochlorous acid was existed as undissociated HOCl and acts as

very strong oxidising property where as OCl- was the dominant species at basic pH which can be

clearely observed from the ionisation curve of HOCl in Figure 5.1215

. It shows that acidic

solution below 6 is more effective for the convertion of Au(I) to Au(III). This was also supported

the effective removal of Au(I) in NaClO medium in the presence of hydrochloric acid by using

CLPC gel as described earlier.

Figure 5.12 Ionization curve of HOCl as a function of pH14

0

10

20

30

40

50

60

70

80

90

1000

10

20

30

40

50

60

70

80

90

100

4 5 6 7 8 9 10 11

% H

yp

och

lori

de

io

n [

OC

l- ]

% H

yp

och

loro

us

aci

d [

HO

Cl]

pH

141

5.4 CONCLUSIONS

The results of this study have demonstrated that cross-linked pure cellulose can be used

as a feed material for high-capacity adsorbent for the recovery of Au(I) from hypochlorite media.

High selectivity of CLPC was observed for Au(I) recovery after oxidation with NaClO from

mixed solution containing other precious metals and base metals. The adsorption closely

followed the Langmuir adsorption isotherm, which indicated that the monolayer adsorption was

involved in the process of gold recovery. Thermodynamic study shows that adsorption of Au(I)

after oxidation was spontaneous and endothermic in nature. Solid state analysis confirmed the

formation of elemental gold particle from the reduction of gold with the aid of CLPC adsorbent.

Thus, it is expected that, CPLC adsorbent could be employed as an excellent and efficient

adsorbent for the recovery of Au(I) after oxidation with NaClO from waste plating solution

142

REFERENCES CITED

1. Farag AB, Soliman MH, Abdel-Rasoul OS, El-Shahawi MS, (2007) Sorption characteristics

and chromatographic separation of gold (I and III) from silver and base metal ions using

polyurethane foams, Analytica Chimica Acta., 601, 218-229.

2. Gönen N, Körpe E, Yıldırım ME, Selengil U, (2007). Leaching and CIL processes in gold

recovery from refractory ore with thiourea solutions. Miner. Eng. 20, 559–565.

3. Cui J, Zhang L, (2008) Metallurgical recovery of metals from electronic waste: a review, J.

Hazard. Mater. 158, 228–256.

4. Zhang H, Ritchie IM, Brooy SRL, (2004) The adsorption of gold thiourea complex onto

activated carbon, Hydrometallurgy, 72, 291–301.

5. Lee HY, Kim SG, Oh JK, (1997) Cementation behavior of gold and silver onto Zn, Al and

Fe powders from acid thiourea solutions, Canadian Metallurgical Quarterly, 3, 149-155.

6. Dahya AS and King DJ, (1983) Developments in Carbon –in-Pulp technology for gold

recovery, CIM Bulletin, 76, 55-61.

7. Niu H, Volesky B, (2003) Characteristics of anionic metal species bio-sorption with waste

crab shells, Hydrometallurgy, 71, 209–215.

8. Zyl PGV, Breet ELJ, Williams CS, (2008) Recovery of KAu(CN)2 from granular activated

carbon using supercritical CO2, J. Supercrit. Fluids, 47, 31–36.

9. Kwak IS, Bae MA, Won SW, Mao J, Sneha K, Park J, Sathishkumar M, Yun, YS, 2010.

Sequential process of sorption and incineration for recovery of gold form cyanide solutions:

Comparison of ion exchange resin, activated carbon and biosorbent, Chemical Engineering

Journal, 165, 440-446.

143

10. Pangeni B, Paudyal H, Inoue K, Kawakita H, Ohto K, Adhikari BB, Alam S, (2012)

Selective recovery of gold using some cross-linked polysaccharide gels Green Chem, 14,

1917-1927.

11. Kosaka Y, (1988) Treatment of Plating Wastewater, pp.47, KAIBUNDO PUBLISHING Co.

Ltd., Tokyo.

12. Dimitrijevic S, Rajcic-Vujasinovic M, Trujic V, (2013) Non-cyanide Electrolytes for gold

plating – A Review, International Journal of Electrochemical Science, 8, 6620-6646.

13. Hasab MG, Rashchi F, Raygan S, (2013) Chloride-hypochlorite oxidation and leaching of

refractory sulfide gold concentrate, Physicochem. Probl. Miner. Process., 49(1), 61-70.

14. McPherson LL, (1993) WATER Engineering and Management, Understanding ORP’s Role

in the Disinfection Process.

144

Part-II

Precious Metals Recovery from Primary and Secondary Sources: Leaching

Followed by Adsorption onto Low Cost Adsorbent

In the first part of the thesis the adsorption followed by recovery of gold among usually

coexisting precious and base metals from model solution was conducted by using the

polysaccharide-based materials as low cost adsorbents. In this part, the recovery attempt of

highly valued precious metals gold and silver from their primary and secondary sources was

described by leaching process followed by adsorption method. This part also deals with the real

application of our technology to be used for recovering from their respective sources by using

low cost adsorbents prepared by very simple method. There are two chapters in this part.

Chapters 6 and 7, the first one is related to the leaching of gold by acidic thiourea solution from

Mongolian gold ore soil sample as primary source and the leached gold was recovered by using

cross linked cotton adsorbent. On the other hand, the Chapter 7 described the leaching of

precious metals such as gold and silver by acidic thiourea solution and their recovery using cross

linked cotton for gold and silver was recovered by zinc cementation method.

145


Leaching of Gold from Mongolian Gold Ore by Acidic Thiourea Solution

Majority of gold in nature was existed in its ore from where it is extracted in the pure form. In

general practice, the fine particles of gold ore after pulverization was treated with some cyanide solution

followed by its recovery using ion exchange resins or activated carbon. Because of high toxicity of

cyanide solution used for the leaching of precious metal like gold, it may be inhaled into our body and

cause serious health hazards so that investigation of less or non toxic leaching agent is required. Literature

showed that thiourea solution leached out some amount of precious metal like silver, gold, platinum and

palladium thus this work was devoted to the preliminary evaluation of the feasibility of thiourea leaching

for the extraction of gold from Mongolian gold ore sample. The quantitative leaching of gold was

achieved by using acidic solution of thiurea from Mongolian gold ore sample. Leached amount of gold

from leached liquor was recovered by using cross-linked pure cellulose (CLPC) gel.

6.1 INTRODUCTION

The cyanide leaching of gold followed by reduction using some reducing agents like Zn

to extract elemental gold from gold bearing ore is one of the commonly used methods, however

due to toxicity of cyanide, new, low cost, non toxic technology has been continuously researched

in this field and developed various extractant for gold such as thiourea, chlorine solution,

hypochlorous acid, hydrogen peroxide, etc. Although a number of gold extractants have been

considered as an alternative to cyanide, the most promising results have been obtained from the

146

thiourea leaching. The gold recovery with thiourea leaching included stages similar to the

conventional cyanidation process such as crushing, grinding, leaching, adsorption, desorption.

The difference between the two processes was based on the principle of using thiourea or

cyanide solution as an extracting agent for gold recovery in leaching. A strong anionic complex

formed with cyanide was seen in alkaline solutions, while a strong cationic complex formation

took place with thiourea in acidic solutions1.

Cyanide leaching2:

4Au + 8CN- + 2H2O + O2 → 4[Au(CN)2]

- + 4OH

- (6.1)

Thiourea leaching:

Au(s) + 2C=S(NH)2(aq) → Au[C=S(NH2)]2+ +e

- (6.2)

Aqua regia approach that was firstly designed to recover gold due to fast dissolution rate

has gained great deal of attention in recent years. However, gold recovery from primary (ores)

and secondary sources (e-waste) by aqua regia leaching was generally applied only in the

experimental research; few full scale operations were conducted because of its strong oxidation

ability and high corrosion to the equipment. In addition, high concentration of acidic wastewater

produced by gold recovery process is very difficult to deal with. Cyanide leaching has been

widely used to recycle gold, however this method exhausts a large amount of toxic cyanide

wastewater, which lead a serious damage to people and the environment, so this method has been

gradually replaced3. Although a higher recovery rate was achieved by the thiosulfate leaching of

gold, a large quantity of leaching reagents should be consumed resulting in a high cost. Leaching

of gold by thiourea has fully taken into consideration due to rapid reaction with gold and low-

price, as well as less environmental impact1, 3

. However, theoretical or applied researches on

147

thiourea leaching for gold processing were not attracted much attention, needing further

investigation and more development. In the present research work, the new to leach Mongolian

ore containing trace amount of gold was successfully achieved by using acidic thiourea solution

and the various experimental parameters were also determined. The leached amount of gold from

the leached liquor was also successfully recovered by using cross-linked pure cellulose (CLPC)

gel.


The Mongolian gold ore soil sample powder was provided by from Mongolia. The total

dissolution test was carried out by using acidic thiourea (0.5 mol dm-3

TU and 0.1 mol dm-3

H2SO4), at the boiling temperature. Then, the dissoluted liquid was filtered and then the contents

of gold and other metals in the filtrate were analyzed by using ICPS-8100 ICP-AES spectrometer.


6.3.1 Distribution of Particle Sizes

To evaluate the particle size distribution in the provided sample of Mongolian gold ore

sample, from 10 g of ore sample was allowed to sieve by using testing sieved and the weight of

ore sample in different size fraction were determined. It was found that in 10 g ore sample, only

0.25 g of the sample had particle size <50 µm, 1.10 g of the sample had particle size >200 µm

and 8.65 g of the sample was in between 50 to 200 µm which corresponded to 2.5, 11.0 and

86.5 % respectively as shown in Figure 6.1. The size fraction less than 50 µm and larger than

200 µm were in small amount therefore in the leaching test, the ore sample containing mixture of

these entire three fraction was used. Table 1 shows the quantitative analysis of metals (mg g-1

)

present in the fresh sample of Mongolian gold ore (mixture of all size particles) leached with

acidic thiourea (0.5 mol dm-3

TU and 0.1 mol dm-3

sulphuric acid).

148

Figure 6.1 Particle size distribution of Mongolian gold ore sample

Table 6.1 Quantitative analysis of metal ions in Mongolian gold ore leached with acidic

thiourea solution

Metals Amount leached mg/g

Au 0.046

Pt 0.018

Al 0.694

Fe 64.75

Co 0.008

Ni 0.040

Cu 0.779

Zn 0.069

0

20

40

60

<50 µm 50-75 µm 75-125 µm 100-150 µm 150-300 µm >300 µm

We

igh

t fr

act

ion

%

Particle size

149

The total amount of precious and base metals leached from the acidic thiourea solution as

presented in Table 1 was evaluated by repeated leaching up to 4 cycles for ensuring complete

leaching of precious metals. For repeated leaching, first of all the ore was leached out by using

acidic thiourea solution then after filtration, the residue was dried and again leached using acidic

thiourea for the second cycle. Similar procedure was repeated until 4 cycles. Among the metal

leached, Au and Pt were present in trace amount in the leaching solution. The cumulative amount

of Au and Pt leaching at different cycle are presented in Figure 6.2(a) and (b). The leached

amounts of gold and platinum increased with increasing leaching cycle from 1 to 2 whereas it

was found to be negligible leaching by using more than 3 cycles. The total amount of Au and Pt

leached (cumulative amount from all 4 cycle) was evaluated as 0.046 and 0.01, mg/g

respectively.

Figure 6.2 Cycle test for the leaching of Au and Pt from Mongolian gold ore sample using acidic

thiourea solution (0.5 mol dm-3

TU and 0.1 mol dm-3

H2SO4). Shaking time = 1 h, shaking speed

= 800 rpm, temperature = 70 °C, weight of gold ore sample = 5 g, volume of solution = 60 cm3

(a)

0

0.01

0.02

0.03

0.04

0.05

0 1 2 3 4

Leaching cycle [-]

Cu

mu

lati

ve

am

ou

nt

of

Pt

leach

ed [

mg g

-1]

0

0.01

0.02

0.03

0.04

0.05

0 1 2 3 4

Leaching cycle [-]

Cu

mu

lati

ve

am

ou

nt

of

Au

lea

ched

[m

g g

-1]

(b) (a)

150

6.3.2 Effect of Leaching Solution

The effect of leaching solution for the leaching of gold is shown in Figure 6.3. Only TU

for leaching of gold had negligible effect, however when acid (0.05 mol dm-3

H2SO4) was added

to the various concentration of TU, the leached amount of gold was also found to be increased

until 0.1 mol dm-3

of TU whereas the further increase of TU concentration had negligible

improvement in gold leaching. The use of only H2SO4 had little bit effect for the leaching of Au

because acid determined the media for chemical species of precious metals as a stable form.

When the TU concentration was kept constant and acid concentration was varied, the leaching

was observed to increase with increasing acid concentration from 0.01 to 0.05 mol dm-3

(Figure

6.3), however on increasing the acid concentration higher than 0.05 mol dm-3

slightly lowered

the leaching efficiency, this was because low concentration of sulphuric acid activated the lone-

pair electrons on TU to facilitate the complexation with gold. On the other hand, high

concentration of acid protonated the TU and impeded the complexation, resulting in the low

leaching4. Thiourea is an organic compound which is soluble in water, and relatively stable in

acidic solutions. The coordination of gold with TU molecule occurred through the sulphur

atoms5. TU complex of gold (Au[C=S(NH2)]2

+) was formed during TU leaching. Similar result

was also observed in the case of Pt as in the case of Au suggesting that the acidic TU solution

containing 0.05 mol dm-3

sulphuric acid and 0.1 mol dm-3

TU was a suitable leaching eluting

agent.

151

Figure 6.3 Effect of leaching solution for gold at different conditions. Shaking time = 24 h,

shaking speed = 150 rpm, at 303 K

6.3.3 Effect of Liquid/solid Ratio for the Leaching of Gold and Platinum

For the evaluation of minimum amount of leaching solution for complete leaching of

targeted metal ion, the volume of acidic TU solution optimized in the previous experiment was

varied keeping ore sample amount constant. Figure 6.4 shows the effect of liquid/solid ratio for

the leaching of gold and platinum at constant TU (0.1 mol dm-3

) and acid (0.05 mol dm-3

)

concentrations. With increasing the liquid/solid ratio from 5 to 20 cm3

g-1

the leaching of gold

and platinum was increased whereas on further increase of liquid to solid ratio above 30 cm3

g-1

,

the amount of leaching of precious metal ions remains almost the constant so based on this result

the 30 cm3

g-1

of liquid/solid ratio for the leaching of gold and platinum was optimized at 0.1 mol

dm-3


H2SO4 concentration.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.1 0.2 0.3 0.4 0.5

Lea

ched

am

ou

nt

of

go

ld [

mg

g-1

]

Leaching solution [mol dm-3]

Thiourea (TU)

Sulphuric acid (H2SO4)

TU at 0.05 M H2SO4

H2SO4 at 0.1 M TU

152

Figure 6.4 Effect of liquid/solid ratio for the leaching of gold and platinum from Mongolian gold

ore at 0.1 mol dm-3

TU, 0.05 mol dm-3

H2SO4 for 24 h at 30°C, Shaking speed = 150 rpm,

sample powder = 1 g

6.3.4 Effect of Contact Time for the Leaching of Gold

Figure 6.5 shows the effect of contact time for the leaching of gold from Mongolian

gold ore soil sample at different temperature. On increasing the contact time from start of the

experiment to until 3 h the leaching amount of metal ions was found to increase and then on

further increasing the contacting time the extracted amount of gold remained almost the same in

all the temperature tested so in order to ensure the complete leaching of these metal ions from

Mongolian gold ore sample, the further experimental work will be conducted until 24 h of

contact time at different temperature.

0

0.01

0.02

0.03

0.04

0.05

0 20 40 60

Am

ou

nt

of

lea

chin

g [

mg

g-1

]

Liquid/solid ratio [cm3 g-1]

Au

Pt

153

Figure 6.5 Effect of contact time for the leaching of gold at 30 cm3

g-1

of solid/liquid ratio using

0.1 mol dm-3


H2SO4 at different temperature at shaking speed of 300 rpm

6.3.5 Elemental Analysis of the Mongolian Gold Ore Sample Before and After Leaching

For the confirmation analysis of complete Au leaching from gold ore sample, the

elemental analysis of sample before and after leaching (residue left after 4 cycles of leaching)

was carried out by EDX spectroscopy. The result is presented in Figures 6.6 (a) and (b). In the

case of gold ore sample in Figure 6.6 (a), the peaks related to C, O, Na, Si, S, K, Ca, Ti, Mn, Fe,

Cu, and Pb were observed together with Au suggesting that the sample contained some amount

of gold. After leaching for 4 cycles (Figure 6.6 (b), the peaks related to Au disappeared from the

spectrum suggesting that gold was completely leached out using acidic thiourea solution. In the

case of platinum, its concentration was very small amount that which may be less than the

detection limit of the EDX spectrometer resulting no peaks even in the sample before leaching.

0

0.009

0.018

0.027

0.036

0.045

0 1 2 3 4

Am

ou

nt

of

lea

ched

go

ld a

nd

pla

tin

um

[mg

g-

1]

t/h

303 K

313 K

323 K

154

The results showed that using thiourea as leaching agent the complete leaching of gold from

trace amount of gold containing ore sample was effectively leached out by this method.

Figure 6.6 Energy dispersive X-ray spectra of (a) Mongolian gold ore soil sample before and (b)

Mongolian gold ore soil sample after leaching with acidic thiourea

0.01

0.1

1

10

0 3 6 9 12

Inte

nsi

ty

keV

C

Au

Na

S

Fe

Fe

Cu

AuAu

M

Mn

K

Si

O Al

Ca

Ti

Pb

0.01

0.1

1

10

0 3 6 9 12

Inte

nsi

ty

keV

CO

Na

Al

Si

S

Ti

K

Mn

Fe

Fe

Cu

Pb

(a)

(b)

155

6.3.6 Recovery of Gold from Leach Liquor Prepared in Acidic Thiourea Solution

In order to recover the trace concentration of gold (0.046 mg g-1

) present in the

Mongolian gold ore soil sample, cross-linked cotton gel was employed as a low cost adsorbent.

After the optimization of TU and sulphuric acid concentrations, the real leach liquor was

prepared using 0.1 mol dm-3


sulphuric acid at 300 rpm at 303 K for 4 h in

order to recover gold. Then, the sample with different dosages of cotton gel varying from 0.5 to

10 g dm-3

were shaken at 303 K for 24 h together with 10 cm3 of pregnant solution and after that

the mixture was filtrate which was then analyzed for Au(III) remaining concentration using ICP-

AES. The result as shown in Figure 6.7 indicated that on increasing the adsorbent dosages from

0.5 to 4 g dm-3

, the efficiency of gold adsorption was found to be increased. However, further

increase in adsorbent dosages from 3 to 10 g dm-3

, almost quantitative adsorption of gold was

achieved. Thus, using 4 g dm-3

, of adsorbent dosages, trace concentration of gold recovery from

real leach liquors from gold ore sample was performed. Such type of real applied result is

promising for the recovery of trace concentration of gold for industrial application by using low

cost adsorbent.

156

Figure 6.7 Effect of solid/liquid ratio for the adsorption of gold using cotton gel from real leach

liquors of Mongolian gold ore. Volume of the liquor = 10 cm3, temperature = 303 K, shaking

time = 24 h, at 150 rpm

6.3.7 Solid State Analysis

The filter cake of CLC adsorbent obtained after the treatment with leached liquor was

first dried in a convection oven at 343K for 24h then it was subjected to solid state analysis by

using digital microscope and X- ray diffraction analyzer as shown in Figures 6.8 (a) and (b),

respectively. From digital photograph of the filter cake in Figure 6.8 (a), it is clear that gold

particles were visually observed on the surface of the adsorbent. Some small gold particles were

observed to be detached from the adsorbent. These particles are possibly elemental gold particle

which was further confirmed from XRD analysis. The XRD spectra of gold loaded CLC

adsorbent is presented in Figure 6.8 (b). The spectra clearly showed the sharp peaks of

0

20

40

60

80

100

0 2 4 6 8 10

% A

dso

rpti

on

of

Au

Solid/Liquid ratio (g dm-3)

157

elemental gold at 2θ values around 37.9, 44.3, 66.2 and 78.3 which are due to the presence of

crystalline elemental gold particles formed after the reduction of gold from leach liquor.

Figure 6.8 Solid state analysis of gold loaded CLC adsorbent (a) digital photograph, and (b)

XRD pattern

6.4 CONCLUSIONS

Acidic thiourea solution, highly efficient non toxic eluting reagent for gold from its

primary sources was investigated. Complete leaching of gold from Mongolian gold ore sample

was achieved by using acidic-thiourea solution which was confirmed form elemental analysis by

EDX spectroscopy. The recovery of gold from leached liquor was successfully achieved by using

cross-linked cotton (CLC) adsorbent. The formations of elemental gold particles after the

treatment of CLC gel was evidenced from digital photograph and X-ray diffraction (XRD)

analysis. Thus, gold can be effectively leached out by using acidic-thioura solution as a non toxic

and environmentally benign and could be expected to employ as an alternative to the toxic

cyanide leaching.

0

1000

2000

3000

4000

5000

6000

10 20 30 40 50 60 70 80

Inte

nsi

ty c

ou

nts

2θ ( Degree)

(b) (a)

158

REFERENCES CITED

1. Li J, Miller JD, (2007), Reaction kinetics of gold dissolution in acid thiourea solution

using ferric sulfate as oxidant, Hydrometallurgy 89, 279-28

2. Lee JD, Concise Inorganic Chemistry (1996), Blackwell Science Pvt. Ltd. 54 University

Street Victoria 3053, Australia.

3. Ying LJ, Li XX,. Quan LW, (2012), Thiourea leaching gold and silver from the printed

circuit board of waste mobile phones, Waste Management 32, 1209-1212

4. Biswas BK, Inoue K, Ohto K, Harada H, Kawakita H, Hoshimo A, (2010), E-waste

management through silver recovery from scrap of plasma T.V. monitors, Proc. Of

International conference on Environmental Aspects of Bangladesh (ICEB10), Japan, Sept.

5. Parker GK, Hope GA, (2008), Spectro-electrochemical investigations of gold leaching in

thiourea media, Mineral Engineering 21, 489-500

159


Recovery of Gold and Silver from Scraps of Plasma TV Monitor

In this chapter an attempt of gold and silver leaching from scraps of plasma TV monitor was conducted

to investigate the alternative and green technology for toxic cyanide leaching which was commonly used

in the industrial processing for precious metal. It was found that dilute acidic thiuorea solutions provided

almost complete extraction of silver as well as gold from scraps of Plasma TV monitors. Biosorbent

prepared from cotton in Chapter 3 exhibited selective capture of gold whereas other precious metals (Pt,

Pd) and base metals were negligibly adsorbed onto the cross-linked cotton (CLC) gel in acidic chloride

media. It was expected that gold present in the leached liquor of plasma TV monitor may be selectively

captured onto this adsorbent so that CLC gel was employed for selective capture of gold from leached

liquor. It was found that gold was selectively captured on this adsorbent from leach liquor, whereas silver

and other base metals remained also in the treated solution. From such a treated solution, the recovery of

silver was examined and it was observed that complete recovery of silver was achieved by using zinc

cementation process.

7.1 INTRODUCTION

Management of metal pollution associated with electrical and electronic waste (e-waste)

is widespread across the globe. Cyanidation has been the predominant process for gold and silver

recovery from mineral resources for more than a century because of its simplicity and economy1-

4. Due to toxicity and environmental concerns, considerable efforts have been made to find

alternative gold and silver lixiviants.

160

Traditionally, cyanide solution was found to be effective leaching agent for precious metal and

widely employed in the industrial processing. Due to some disadvantages and toxicity of cyanide

solution which would described later in detailed, the investigation of non-toxic and low-cost

technology has been required from the view point of the protection of human and environmental

health. Therefore, this work is of great significance for over state, and at the same time it can

bring economic benefits to the industrial applications. At present, there are a lot of techniques of

recycling waste scraps in the world. Leaching is the initial step in a hydrometallurgical process;

it is also the most important key during the precious metal recovery from waste scarps5. Silver is

one of the widely used metals in industry. The majority of silver is consumed in film processing

industry with 40–50% of overall consumption, electrical industry follows with 20–30%, and

ornaments and jewelry consist of 10% of overall consumption. In addition to the recovery of

silver from scrap metals, silver is produced either directly from ores or as a by-product during

the productions of zinc, copper and lead6. Depletion of natural resources has been becoming a

serious issue in recent decades. This situation gives an arising interest to recover recyclable

resources from industrial waste. Recovery and recycling of silver from wastes not only ensure

resource conservation, but also at the same time minimize the detrimental effects of ore mining

to the environment. Scraps of plasma TV monitors, an emerging-waste, is thus thought to create

an environmental effect due to its massive disposal. Hence recovery of silver from scrap of

plasma TV monitors is of great concern.

In hydrometallurgical process, leaching is usually the first step in recovering metal from solid

waste. Literature showed that TU solution can be functioned as an effective leaching agent in the

acidic medium. The TU is an effective and promising non-conventional leaching agent for the

recovery of precious metals. Under acidic conditions, TU reacts rather selectively with silver and

161

gold, thus resulting in the formation of cationic complexes (Ag[CS(NH2)2]3+). The leaching of

precious metals by acidic TU solution followed by its recovery using some biomass based low -

cost adsorbent can be more promising to develop a green and environmentally benign technology

for the recovery of precious metals from various e-wastes like scraps of plasma TV monitor.

Even though cyanide leaching is widely used on a commercial scale, this process has several

well-known disadvantages: low leaching rates (24-72 h contact times), high toxicity of cyanide,

and severe environmental restrictions. The process must be carefully monitored and controlled,

and the spent cyanide leaching solutions must be treated before being discharged to the

environment. Because of these drawbacks, the utilization of several alternative lixiviants to

cyanide such as TU was reported for the treatment of scraps containing precious metals like

silver and gold. The kinetics of the precious metal leaching with TU was much faster than those

of alkaline cyanidation. High consumption of TU and acid concentrations is a weak point of the

process. Therefore, the optimization of the process is necessary.

7.1.1 Advantages of Thiourea Leaching Over Cyanide Leaching

This method was developed as a potential substitute for cyanidation (of gold as well as

silver). The use of TU, as an extracting agent for precious metals has shown promise for the

implementation in various metallurgical areas7. Lower toxicity of TU and greater dissolution rate

of gold and silver compared to cyanide give it an advantage in reaching commercial application

before other non-commercial lixiviants. Alternative lixiviants include bromides (acid and

alkaline), chlorides and thiosulfate. The lower sensitivity to base metals (As, Cu, Zn, Pb, Sb) or

impurities render possible the use of this process on many refractory gold ores. The leaching test

162

of gold and silver with TU showed that TU had several advantages over conventional

cyanidation.


The scraps of plasma TV monitor in powdered form was provided by Nishinihon Kaden,

Recycle Corporation, Kitakyushu, Japan and was stored in desiccators at room temperature until

used. The particle size used was as received i.e. less as 100 µm.

In the preliminary experiment it was found that the aqua regia solution and concentrated

hydrochloric acid solution were remarkably effective for the complete dissolution of base metal

ions from the scrap of plasma TV monitor whereas mixed solution of sulphuric acid and TU was

observed to be effective for the dissolution of precious metal like gold and silver. Thus, in order

to find out the total amounts of metal ions present in the sample of scraps of plasma TV monitor,

dissolution test was carried out by using acidic TU (0.05 mol dm-3


H2SO4),

concentrated hydrochloric acid (11.6 mol dm-3

) and aqua regia at their boiling temperature using

1 g of sample powder in 30 cm3 of leaching agent for 4h. Then, the dissoluted liquid was filtered

and finally the silver along with other metals content of the filtrate was analyzed by using ICPS-

8100 ICP/AES spectrometer. Our target is recovery of precious metals like gold and silver after

their complete dissolution from the scraps of plasma TV monitor so that TU and sulphuric acid

concentration was optimized prior to prepare the acidic TU solution. For the determination of

optimal TU solution, the leaching test was carried out by varying the concentration of TU

solution (0.001 to 0.2 mol dm-3

) at liquid/solid ratio of 10 to 500 cm3 mg

-1 keeping sulphuric acid

concentration constant. Similarly, in the another set of experiment, the sulphuric acid

concentration was changes from 0.01 to 2 mol dm-3

keeping thiourea concentration constant to

163

find out the optimum concentration of sulphuric acid at liquid/solid ratio of 500 cm3 mg

-1 at

303K. The dissoluted liquid was filtered and analyzed for gold and silver along with other metals

content by using ICP/AES. The selective recovery of gold was carried out by using cross-linked

cotton gel at different sold liquid ratio ranging from 0.5 to 10 g dm-3

. After selective recovery of

gold by cotton gel, silver remained in the leaching solution together with other base metal from

where silver was recovered by Zn cementation at different solid/liquid ratio ranging from 0.5 to

10 g dm-3

. The digital photograph and XRD of gold-loaded cotton and silver-loaded Zn powder

were recorded by using digital microscope and X-ray diffraction spectrophotometer, respectively.


7.3.1 Evaluation of Suitable Leaching Agents

In order to find out the effective leaching solution for gold and silver from scrap of

plasma TV monitor, three different leaching solutions namely aqua regia, concentrated

hydrochloric acid (11.6 mol dm-3

) and acidic TU. The amount of the leached metals ions

including gold and silver using three different leaching agents were tabulated in the Table 7.1.

Acidic TU acted as an effective leaching agent for precious metals like gold and silver compared

to other metal ions. The aqua regia and concentrated hydrochloric acid were less effective than

acidic TU for gold and silver leaching compared to other metals ions.

164

Table 7.1 Quantitative analysis of scraps of plasma TV monitor by various leaching

agents

Leached amount (mg g-1

)

Metals Acidic thiourea HCl Aqua regia

Ag 0.578 0.438 0.453

Al 2.726 2.811 0.995

Fe 0.101 0.073 0.051

Cu 0.189 0.121 0.126

Zn 0.352 0.151 0.242

Pb 0.149 10.27 10.32

Au 0.152 0.009 0.005

Pt 0.081 0 0

Pd 0.048 0.043 0

Ca 4.418 2.836 3.030

Sb 0.020 0.036 0.513

7.3.2 Optimization of Leaching Condition from Plasma TV Monitor

7.3.2.1 Effect of thiourea concentration for leaching of silver

Figure 7.1 shows the effect of thiourea concentration at varying liquid/solid ratio for

leaching of silver from scraps of plasma TV monitor. The results showed that on increasing the

thiourea from 0.001-0.01 mol dm-3

, the leaching amount was found to be increased, whereas

further increasing concentration of TU from 0.01 to 0.2 mol dm-3

the leaching amount was not

so much drastically changed or it remained constant regardless of liquid/solid ratio. So based on

these results, the concentration of 0.1 mol dm-3

TU was optimized for further experimental work

at 500 cm3

g-1

of liquid/solid ratio using 0.05 mol dm-3

sulphuric acid concentrations.

165

Figure 7.1 Leaching behavior of silver at different thiourea concentration as a function of

liquid/solid ratio. Conditions: weight of the powder used = 100 mg, sulphuric acid = 0.05 mol

dm-3

, shaking time = 24 h, temperature = 303 K

7.3.2.2 Effect of suphuric acid concentration for leaching of silver

The effect of acid concentration on silver extraction is illustrated in Figure 7.2. An

increasing in the acid concentration caused a noticeable decrease in the extraction of silver at 0.1

mol dm-3

TU concentrations at liquid/solid ratio of 500 cm3

g-1

. It was seen that the maximum

amount of silver leached was achieved at acid concentration of 0.05 mol dm-3

. Thus, this less

acid concentration used for leaching of silver was feasible from the economic point of view. So,

the optimized acid concentration for further experimental work was determined to be 0.05 mol

dm-3

.

Am

ou

nt

extr

act

ion

of

Ag [

mg

g -1

]

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400 500

0.2 M T.U.

0.1 M T.U.

0.05 M T.U.

0.04 M T.U.

0.02 M T.U.

0.01 M T.U.

0.005 M T.U.

0.003 M T.U.

0.001 M T.U.

Liquid/solid ratio [mg g-1]

166

Figure 7.2 Leaching behavior of silver as a function of acid concentration. Conditions: weight of

the powder used = 100 mg, thiourea = 0.1 mol dm-3

, liquid/solid ratio = 500 cm3

g-1

, shaking time

= 24 h, temperature = 303 K, shaking speed = 150 rpm

7.3.3 Acidic Thiourea Leaching

It was found that 0.05 mol dm-3

of sulphuric acid and 0.1 mol dm-3

of TU solution were

optimized to be effective for the leaching of higher amount of gold and silver from scrap of

plasma TV monitor. So that, the mixed solution containing optimized concentration of sulphuric

acid and TU was used for the leaching of metal ions especially silver and gold.

7.3.3.1 Effect of liquid/solid ratio

Figure 7.3 shows the leaching of various metal ions from scraps of plasma TV monitor

by using acidic TU solution as a function of liquid/solid ratio. The leaching of metal ions

increased with increasing liquid/solid ratio below 30 cm3

mg-1

where more than 95% of gold and

silver were leached out. The total leached amount of silver and gold using acidic TU solution

were evaluated to be around 0.56 and 0.14 mg g-1

respectively, extracted using acidic TU

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.4 0.8 1.2 1.6 2

H SO Concentration (M)

Am

ou

nt

extr

act

ion

of

Ag [

mg

g -1

]

[H2SO4]/mol dm-3

167

solutions which were nearly the 100% of the total amount of silver and gold in the sample of

plasma TV monitor. Thus, it was optimized that the acidic TU solution at liquid/solid ratio of 50

cm3

mg-1

was optimized for complete leaching of gold and silver from the scrap of plasma TV

monitor. It was also seen that calcium and aluminium were also leached by acidothiourea

solution under the experimental conditions.

Figure 7.3 Leaching behavior of various metals ion as a function of liquid/solid ratio. Thiourea =

0.1 mol dm-3

, weight of powder leachate = 100 mg, sulphuric acid = 0.05 mol dm-3

, temperature

= 303 K, shaking time = 24 h, shaking speed = 150 rpm

0

0.5

1

1.5

2

2.5

3

3.5

4

15 20 25 30 35 40 45 50

Am

ou

nt

ex

tra

ctra

tio

n o

f m

eta

l io

ns

[mg

g-1

]

Liquid/Solid ratio [cm3 mg-1]

Ag

Au

Pd

Pt

Al

Ca

Fe

Ni

Cu

Zn

Sn

Pb

168

7.3.3.2 Effect of temperature for leaching of silver

The effect of temperature for the leaching of silver using acidic TU solution is shown in

Figure 7.4. With increasing the temperature from 303 K to 323 K the dissolution rate of silver

by acidic TU solution was only slightly affected. The noticeable difference in the rate of silver

leaching was not observed even with the increase of temperature of the system.

Figure 7.4 Leaching kinetics of silver at different temperatures. Liquid/solid ratio = 200 cm3

g-1

,

thiourea (TU) = 0.1 mol dm-3

, H2SO4 = 0.05 mol dm-3

, shaking rate = 600 rpm

7.3.4 Recovery of Gold and Silver from Leached Liquor Prepared from Acidic Thiourea

7.3.4.1 Recovery of gold using cross-linked cotton gel

Acidic TU leached liquor of plasma TV monitor scrap contained trace amount of gold as

well as silver. It was found that the cotton cellulose after cross-linking with concentrated

sulphuric acid was very much effective for the recovery of Au(III) as described in Chapter 3. In

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6

Am

ou

nt

ex

tra

ctio

n o

f si

lve

r /m

g g

-1

t/h

303 K

313 K

323 K

169

the present work, an attempt has been conducted to recover the trace concentration of gold by

using cross-linked cotton gel as an adsorbent from acidothiourea leach liquor of plasma TV

monitor scrap. The real leach liquor for recovering gold contained in plasma TV monitor scrap

was prepared using 0.1 mol dm-3


H2SO4 at 600 rpm at 303 K for 4 h at the

liquid/solid ratio of 50 cm3

mg-1

. After filtration the constituent of metal ions present in leach

liquor were analyzed to be Ag: 1.907 mg dm-3

, Au: 0.490 mg dm-3

, Pt: 0.252 mg dm-3

, Pd: 0.168

mg dm-3

, Al: 9.084 mg dm-3

, Ca: 14.729 mg dm-3

, Fe: 0.342 mg dm-3

, Cu: 0.631 mg dm-3

, Zn:

1.169 mg dm-3

, Sb: 0.067 mg dm-3

, and Pb: 0.498 mg dm-3

by ICP-AES which corresponded to

Ag: 0.572, Au: 0.147, Pt: 0.075, Pd: 0.050, Al: 2.725, Ca: 4.418, Fe: 0.102, Cu: 0.189, Zn: 0.350,

Sb: 0.020 and Pb: 0.149 mg g-1

respectively. Then it was treated with different dosages of cotton

gel varying from 0.5 to 10 g dm-3

and shaken for 24 h at 303 K together with the 10 cm3 of liquor

solution and after that the mixture was filtrate the solution was analyzed for gold remaining

concentration using ICP-AES.

Figure 7.5 shows the adsorptive removal of gold from leached liquor using CLC gel as a

function of solid/liquid ratio. The result showed that the gold was selectively adsorbed onto the

CLC gel whereas silver and other base metals were negligibly or insignificantly adsorbed by this

adsorbent. The result as shown in Figure 7.5 depicted that on increasing the adsorbent dosages

from 0.5 to 3 g dm-3

, the efficiency of gold adsorption was found to be increased. However,

further increase in adsorbent dosages from 3 to 10 g dm-3

; almost quantitative adsorption of gold

was achieved. Thus, using 3 g dm-3

of adsorbent dosages, effective recovery of trace

concentration of gold from real leach liquors of plasma TV monitor scraps was successfully

achieved. After selective adsorption of gold by CLC gel, majority (1.887 mg dm-3

) of silver

170

remained in the leached liquor together with other base metals from which silver was recovered

by zinc cementation which would be described in later section.

Figure 7.5 Effect of solid/liquid ratio for the adsorption of gold from real leach liquors of plasma

TV monitor scraps using cross-linked cotton gel. Volume of the liquor = 10 cm3, temperature =

303 K, shaking time = 24 h, shaking speed = 150 rpm

7.3.4.2 Recovery of silver by cementation with zinc powder

Cementation or metal displacement is an electrochemical process in which a more noble metal

ion is recovered from an aqueous or targeted pregnant solution by a less noble precipitant. The

reaction has been widely used in hydrometallurgical processing for the recovery of silver and

gold. Silver recovery by cementation, an inexpensive and easy method to deposit silver in its

metal forms was conducted without adjusting the pH of the pregnant solution after the treatment

with cotton gel to selectively recovered gold ion at solid/liquid ratio of 5 g dm-3

. After the

treatment with cross-linking cotton gel, the concentration of different metal ions evaluated in the

filtrate was Ag: 1.887 mg dm-3

, Au: 0.000 mg dm-3

, Pt: 0.212 mg dm-3

, Pd: 0.162 mg dm-3

, Al:

0

20

40

60

80

100

0 2 4 6 8 10

% A

dso

rpti

on

of

Au

Solid/Liquid ratio (g/L)Solid/liquid ratio/g dm-3

171

8.624 mg dm-3

, Ca: 14.329 mg dm-3

, Fe: 0.308 mg dm-3

, Cu: 0.531 mg dm-3

, Zn: 1.050 mg dm-3

,

Sb: 0.062 mg dm-3

, and Pb: 0.406 mg dm-3

by ICP-AES which corresponded to Ag: 0.566, Au:

0.000, Pt: 0.063, Pd: 0.048, Al: 2.587, Ca: 4.298, Fe: 0.092, Cu: 0.159, Zn: 0.315, Sb: 0.018 and

Pb: 0.121 mg g-1

, respectively. This solution was used for cementation experiment. To elucidate

the cementation of silver from silver-containing acidic TU leach liquor (0.1 mol dm-3

TU and

0.05 mol dm-3

H2SO4), various amounts of zinc powder were added to the fixed volume of the

above mentioned liquors (10 cm3), stirred at 150 rpm at constant temperature for 24 h. The silver

concentration in the leached liquor before and after the treatment with Zn powder was analyzed

by ICP/AES. The reacted solution was filtered and silver concentration in the filtrate was

measured for and the result is presented in Figure 7.6. The % removal of silver from leached

liquor was found to be increased with increasing solid/liquid ratio and only a small dose of zinc

powder was found to be effective to recover silver from leach liquor. The optimal solid/liquid

ratio was determined to be 8 g dm-3

. The difference in the oxidation-reduction potential (ORP) of

silver i.e.( +0.80 V) and zinc (-0.76 V) may be considered to cementation of silver from trace

concentration of silver containing acidic-thiourea leach liquor. By using zinc powder, 99.28 % of

silver recovery was achieved by a simple process. The inferred reaction for the cementation of

silver is shown by Eq. (7.1). Again, to characterize the cemented silver particles in the residue

after filtration, it was dried and used for solid state analysis.

2 Ag[CS(NH2)2]3+ + Zn

0(s) 2 Ag

0(s) + Zn-thiourea complex (7.1)

172

Figure 7.6 Recovery of metallic silver by cementation with zinc powder at varying solid/liquid

ratio. Conditions: Volume of acidothiourea leach liquors = 10 cm3, temperature = 303 K, shaking

time = 24 h, shaking speed = 150 rpm

7.3.5 Characterization of the Gold Loaded Cotton and Cemented Silver

For characterization of the cemented silver, XRD and optical microscope images

were recorded. XRD data are presented in Figure 7.7 which showed that pure crystalline silver

were cemented on the zinc powder (at 2θ values of 32.1, 38.6, 43.1, 64.4 and 77.5). Similar

peaks were observed by Chunhua et.al., 20078 and Khan et. al., 2011

9 for pure silver nano

particles and Abdel-Aal and Farghaly, 200710

for cemented silver particle. Figures 7.8 (a) and

(b) shows the optical microscope images of the cemented silver at different magnifications. The

white shining particles observed on the surface of Zn powder were silver particle. Various sizes

of silver particles were cemented on the zinc powder. Moreover, Figure 7.8 (c) shows the digital

photograph of cross-linked cotton gel after the treatment with leached liquor of the plasma TV

monitor scraps. The golden colored metallic particles of elemental gold were observed on the

0

20

40

60

80

100

0 2 4 6 8 10

% a

dso

rpti

on

of

Ag

S/L ratio [g dm-3]

surface of cotton gel suggesting that gold was effectively ad

into its elemental form which was further confirmed from the XRD spectrum of gold

cotton gel as shown in Figure 7.8 (d)

observed at 2θ values of 39.12, 4

cotton after treatment with leached liquor which demonstrated that the gold from leached liquor

of plasma TV monitor was recovered at its elemental form without using any type of reducing

agent by the aid of cross-linked cotton gel.

Figure 7.7 XRD patterns for metallic silver cemented on zinc powder

surface of cotton gel suggesting that gold was effectively adsorbed and simultaneously reduced

into its elemental form which was further confirmed from the XRD spectrum of gold

Figure 7.8 (d). It showed that the crystalline peaks of the pure gold were

values of 39.12, 44.64, 66.21 and 78.15 in the XRD spectrum of cross


monitor was recovered at its elemental form without using any type of reducing

linked cotton gel.

XRD patterns for metallic silver cemented on zinc powder

173

sorbed and simultaneously reduced

into its elemental form which was further confirmed from the XRD spectrum of gold-adsorbed

. It showed that the crystalline peaks of the pure gold were

4.64, 66.21 and 78.15 in the XRD spectrum of cross-linked


monitor was recovered at its elemental form without using any type of reducing

174

Figure 7.8 Optical microscope image of metallic silver cemented on zinc powder (a) 150x

magnifications, (b) 300x magnifications and (c) digital photograph of CLC gel after the

treatment with leach liquor and (d) XRD pattern of gold-loaded cotton gel

(c)

(c)

0

200

400

600

800

1000

1200

1400

1600

1800

10 30 50 70

Inte

nsi

ty c

ou

nts

2 theta [ degree]

(d)Au (c)

175

7.4 CONCLUSIONS

Selective recovery of gold from of plasma TV monitor scraps by acidic TU leaching

followed by adsorption onto cross-linked cotton gel together with remaining silver was achieved

by zinc cementation method. The effect of various parameters for the optimization of the

conditions on the process such as initial concentration of TU and acidity in the solutions as well

as reaction temperature were systematically examined. It was confirmed that the initial

concentration of TU and acidity in the solution affected the dissolution rate in some extent for

leaching of silver. The dissolution rate of the process was not affected by increasing temperature

from 303 to 323 K. The optimal operations were conducted using acidic TU solution for the

plasma TV monitor scraps. The white shining particles of silver were recovered on the surface of

Zn powder. Visually observed golden colored metallic particles observed on the surface of

leached liquor treated cross-linked cotton were confirmed to be pure metallic gold particle by

XRD analysis.

176

REFERENCES CITED

1. Li J, Safarzadeh MS, Moats MS, Miller JD, LeVier KM, Dietrich M, Wan RY, (2012)

Thiocyanate hydrometallurgy for the recovery of gold: The leaching kinetics, 113-114, 10-

18.

2. Yang X, Moats MS, Miller JD, (2010a), Gold dissolution in acidic thiourea and thiocyanate

solutions. Electrochim. Acta 55, 3643 – 3649.

3. Yang X, Moats MS, Miller JD (2010b) The interaction of thiourea and formamidine di-

sulfide in the dissolution of goldinsulphuricacidsolutions.Miner.Eng.23 (9), 698 – 704.

4. Yang X, Moats MS, Miller JD (2010c), Usingelectrochemical impedance spectroscopy to

investigate gold dissolution in thiourea and thiocyanate acid solun. ECS Trans. 28 (6), 213–

222.

5. Zhang Y, Liu S, Xie H, Zeng X, Li J, (2012) The 7th

International Conference on Waste

Management and Technology Current status on leaching precious metals from waste printed

circuit boards, Procedia Environmental Sciences 16, 568.

6. Oncel MS, Ince M, Bayramoglu M, (2005) Leaching of silver from solid waste using

ultrasound assisted thiourea method, Ultrasonics Sonochemistry 12:237–242.

7. Chunhua L, Xiupei Y, Hongyan Y, Zaide Z, Dan X, (2007) Preparation of silver

nanoparticle and its application to the determination of ct-DNA, Sensors 7, 708-718.

8. Khan MAM, Kumar S, Ahamed M, S Alrokayan SA, AlSalhi MS, (2011) Structural and the

rmal studies of silver nanoparticles and electrical transport study of their thin films,

Nanoscale Research Letters 6, 434.

177

9. Abdel-Aal EA, Farghaly FE, (2007) Preparation of silver powder in micron size from used

photographic films via leaching-cementation technique, Power Technology 178, 51-55.

10. Chunhua L, Xiupei Y, Hongyan Y, Zaide Z, Dan X, (2007) Preparation of silver

nanoparticle and its application to the determination of ct-DNA, Sensors 7, 708-718.

178


Concluding Remarks and Future Outlook

8.1 CONCLUDING REMARKS

The increase in industrial demand for the precious metals has brought forth a steady

growth in the need for refining of the precious metals in cost effective technique. In addition, the

precious metals are enormously used for jewelry, decoration dentistry and in many sophisticated

instruments including thermocouple elements. Gold is one of the most important precious metals

due to its huge variety of applications in industry and economic activity. This creates a

compelling reason for developing more efficient and environment-friendly methods for the

adsorption of gold together its recovery in metallic form from aqueous acidic chloride media and

also from industrial waste streams. A number of methods such as adsorption on bio-adsorbents,

ion exchange resins, solvent extraction and Au(III) precipitation with the help of reagents have

widely utilized for recovery of Au(III) from aqueous solution. Due to the several difficulties and

disadvantages of these recovery processes such as causing environmental pollution, poor

selectivity, require much more labor and time as well as high cost. Nowadays therefore, interest

in simple, robust, easy-to-use, selective and efficient innovative methods for the recovery of

precious metals has been increasing. The polysaccharide-based adsorbents with high selectivity

and remarkably high uptake capacity of Au(III) are suitable candidates in adsorption process to

full-fill such requirements in solid/liquid system. The polysaccharide biomaterial with hydroxyl

functional groups are responsible for the adsorption of anionic chloro-complex of Au(III) in

acidic chloride media. The treatment with concentrated sulphuric acid makes the polymer

179

matrices more strong and converts to the amorphous form by creating the C-O-C ether bond

linkage which together with many hydroxyl groups in the polysaccharide materials are quite

responsible for the adsorption of Au(III) by coordination reaction.

In the present research, Au(III) recovery was conducted from commercial cellulose by

cross-linking with concentrated sulphuric acid. The research work was conducted with aiming

the development of new adsorbents which successfully adsorbed and subsequently reduced the

Au(III) to its elemental form without adding any types of reducing agents. For that,

commercially available cellulose powder was first cross-linked with concentrated sulphuric acid

in order to create the new coordinating sites for Au(III) adsorption. During the treatment,

crystalline form of commercial cellulose which have very low affinity with Au(III) ion was

converted into amorphous form with very high affinity for Au(III) ion. The trivalent gold was

selectively adsorbed onto the cross-linked cellulose gel from the mixture of other precious and

base metals. It was visually observed that golden colored shining metallic particle was formed

and floating at the surface of the reactor at the short contact where as it was found to be

aggregated then forming heavy particle after long time contact which was settled down on the

bottom later. It was further confirmed from the observation of crystalline peaks of elemental gold

[reduced gold Au(0)] in XRD spectrum Au(III)-loaded cross-linked cellulose gel . So, based on

the results, we further expected to attempt whether such type of quantitative and selectiveness of

gold adsorption was common to all kinds of polysaccharide or not, we employed commercially

available polysaccharides such as dextran, alginic acid and pectic acid with hydroxyl as a major

functional one by cross-linking with concentrated sulphuric acid similar to the case of cellulose

gel. In all the cases the cross-linked product exhibited high selectivity for Au(III) ion from the

mixed solution containing other precious and base metals thus the polysaccharide-based

180

adsorbent investigated in this study are expected to be promising materials for large scale

industrial processing of gold recovery.

Since, the commercial cellulose after cross-linking with concentrate sulphuric acid yield

very effective adsorbent for Au(III) thus we have tried to extent our technology for the other low

cost cellulose rich biomass. Thus adsorbents were prepared from cellulose rich cotton and paper

by the similar method of cross-linking reaction with sulphuric acid. Original cotton and paper

has poor or negligible adsorption capacity and low physical stability because attachments of all

three hydroxyl groups in the same ring may cause steric hindrance and also the hydroxyl groups

are not easily accessible to chemical reaction. Chemical modification such as cross-linking by

condensation reaction using concentrated sulphuric acid into raw cotton develops physical

stability of gel and possesses high adsorption capacity. In the cotton before treating with

concentrated sulphuric acid, there were light compounds which made the materials floating in the

solutions when cross-linked with concentrated sulphuric acid it makes the compounds heavier

and no floatation in the solution during the adsorption. In addition, there was the conversion of

low molecular weight compound into high molecular compound after treating with concentrated

sulphuric acid. From our investigation, it was found that the crystalline structure of cotton was

changed into amorphous after cross-linking condensation reaction with concentrated sulphuric

acid. The experimental results of cotton gel showed that Au(III) was highly and effectively

selective over the studied precious metals such as Pt(IV), Pd(II) and base metals like Cu(II),

Zn(II), Fe(II) and Ni(II) until 2 mol dm-3

concentration of hydrochloric acid. The selectiveness

and effectiveness of Au(III) adsorption was also the same in the results of actual solution

obtained from Tanaka Kikinzoku i.e. chlorine containing hydrochloric acid. This results gave the

strong evidence for the utilization of our adsorbent in real application for the recovery of Au(III)

181

from mixture solution of other precious and base metals. The kinetic results demonstrated that

Au(III) uptake by cotton gel was found to increase with increasing the temperature of the

adsorption system showing exothermic nature of adsorption process. The activation energy

evaluated from the study at different temperature was found to be 78.8 kJ mol-1

showed chemical

nature of adsorption. The maximum loading capacity of Au(III) was 6.21 mmol g-1

which was

high as compare as other adsorbents. Interestingly, the adsorbed Au(III) was reduced into the

elemental gold in the cotton gel as evidenced from the XRD analysis, digital photograph and also

from visual observation as well. Further the more abundantly distributed waste paper was

utilized for the recovery of gold together with the mixture solution of other precious and base

metals from acidic chloride media. Both the materials after cross-linking with concentrated

sulphuric acid selectively adsorbed gold with extremely high adsorption capacity. Comparing

with cotton, waste paper was much cheaper and abundant. So we were intended to elaborate our

technology to more abundant cellulose rich material like paper because Japan is the third largest

country for paper production in the world thus raw paper for adsorbent preparation was easily

available in low cost in Japan in comparison to cotton biomass. The cross-linked paper gel was

prepared by the same method for testing in the selectivity of Au(III) which was more cheaper

than cotton. From all the adsorption tests the results and the conclusions are described below in

point wise forms:

1. The main involvement of the hydroxyl functional groups for Au(III) adsorption and its

simultaneous reduction was elucidated by comparing the adsorption-reduction behavior

of various types of adsorbents prepared from different types of commercial

polysaccharides such as pure cellulose, dextran, pectic acid and alginic acid that all

182

containing common hydroxyl group as a major functional group but having different

structures.

2. It was concluded that new structure of C-O-C linkage was formed in all types of cross-

linked polysaccharides after cross-linking reaction with sulphuric acid.

3. In all 4 types of polysaccharide gels, very high selectivity towards Au(III) at whole range

of hydrochloric acid concentration tested and visual observation of the formation of

shining gold on the surface of reactor vessels were observed that were confirmed the

similar mechanism for all types of cross-linked polysaccharides adsorbents. Moreover,

the peaks related to carboxyl functional group were formed or in some cases like alginic

acid or pectic acid gel its intensities were increased after the treatment of all of these

polysaccharide gels with Au(III) solution which indicated the conversion of some

hydroxyl group into carboxyl one. It was inferred that the oxidation of hydroxyl groups

of these adsorbents to convert carbonyl groups resulted in the release of electron which

were utilized for Au(III) reduction.

4. It was concluded that adsorption isotherm studies was the Langmuir type of monolayer

adsorption.

5. The results of kinetic study for cross-linked cellulose gel showed that the rate of

adsorption of Au(III) was improved on increasing the temperature of the adsorption

system.

6. The value of activation energy at different temperature for Au(III) adsorption on cellulose

gel was evaluated as 71.8 kJ mol-1

which suggested that the adsorption of Au(III) onto

cellulose gel was a chemical adsorption and its positive value indicated that increased

temperature favored the adsorption process.

183

7. The natural bio-polymer contained in a cotton-rich cellulosic material was a novel

material for selective adsorption followed by reduction of Au(III) by simple cross-linking

condensation reaction with concentrated sulphuric acid.

8. High selectivity of cross-linked cotton (CLC) gel was confirmed from the adsorption test

of gold from the mixture of other precious metals likes platinum, palladium and base

metals such as Zn, Ni, Fe, Cu, Na in hydrochloric acid medium. More or less quantitative

adsorption of Au(III) until 3 mol dm-3

of hydrochloric acid concentration (0.1 to 3 mol

dm-3

) whereas insignificant or negligible adsorption of base and other precious metals

showing high selectivity for Au(III) adsorption.

9. Quantitative removal of Au(III) from acidic chloride media was achieved from the

solution containing trace concentration of Au(III) thus it was found to be effective

adsorbent for gold. An energy dispersive X-ray analysis of the filter cake obtained the

adsorption of Au(III) by CLC gel showed the appearance of new peaks of gold in

addition to the peaks of CLC gel which was confirmed the effective Au(III) adsorption on

this adsorbent.

10. The intense crystalline peaks of the feed material (cotton) disappeared after cross-linking

with sulphuric acid suggesting that crystalline form of cotton cellulose was converted into

amorphous form. The amorphous adsorbents prepared by acid treatment was effective for

Au(III) adsorption than its native (crystalline) form.

11. Formation of thin layer of yellow shining metallic aggregate on the surface of reacting

bottle after shaking CLC gel with Au(III) solution was confirmed the reduction reaction

of gold to its elemental form. It was also further confirmed from microscopic analysis

using optical electron microscope. In addition, X-ray diffraction spectroscopic analysis of

184

gold loaded CLC gel showed the presence of the crystalline peaks of elemental gold at

38.08, 44.34, 64.56 and 77.48 of 2ɵ values that was confirmed with the reduction

reaction of Au(III) with the aid of CLC gel.

12. More cheaper adsorbent for Au(III) was investigated by using filter paper as a feed

material for adsorbent by the same method described in the case of cotton preparation.

The selectivity of cross-linked paper (CLP) for Au(III) was also found to be high and

forms the similar types of gold aggregate during the adsorption process.

13. Kinetic studies of Au(III) onto CLP gel at different temperature showed improvement in

the rate of Au(III) removal with increasing temperature. The evaluated value of activation

energy showed that the adsorption reaction of Au(III) onto CLP gel was mostly

controlled by chemical reaction.

14. The incineration reaction of the cross-linked paper before and after gold-loading left the

residue of 0.75 % and 30 % of the ash which was concluded that 29.25 % of the gold-

loaded sample was composed of the loaded amount of gold. The negligible amount

(<1%) of weight remain % in the case of the unloaded sample showed that nearly pure

Au(0) was recovered from Au(III) solution by using CLP gel.

15. The successful recovery of Au(I) from gold-sulphite solution in hypochlorite media was

achieved by using cross-linked pure cellulose (CLPC) gel which suggested high

possibility of the application of CLPC gel for the recovery of gold from Au(I) rich

effluents generated from spent of cyanide gold plating and leaching factories.

16. Recovery of gold from Mongolian gold ore and silver from scraps of plasma TV monitor

was performed by leaching process using dilute thiourea solution

185

17. The adsorbed gold and silver were recovered by using low cost adsorbents from

polysaccharide materials such as cross-linked cellulose gel and cross-linked cotton gel,

respectively.

The polysaccharide-based adsorbents with high selectivity and with remarkably high uptake

capacity of Au(III) were investigated in this research work.

8.2 FUTURE OUTLOOK

Based on the above mentioned results of the present work, the future outlook is summarized onto

the following numbers:

1. The recovery of other precious metals like platinum and palladium will be conducted by

developing the suitable polysaccharide adsorbents after immobilization of nitrogen

containing functional groups.

2. The recovery attempt of Au(I) will be applied in the real spent solution of cyanide by our

simple technology.

186

Appendices

187

Appendix I

List of Published Papers

1. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, Biplob

Kumar Biswas, Shafiq Alam, Optimization of adsorption process for tetrafluoroborate

removal by zirconium(IV)-loaded orange waste gel from aqueous solution, Environ. Technol.,

33 (2012) 845-850.


Kumar Biswas, Shafiq Alam, Adsorptive separation of tetrafluoroborate from aqueous

solution by zirconium(IV)-loaded saponified orange juice residue, Proceeding, 19th

Japan-

Korea Symposium on Water Environment, (2010) Page 89-96.

3. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, Shafiq

Alam, Selective recovery of gold(III) using cotton cellulose treated with concentrated

sulphuric acid, Cellulose, 19 (2012) 381-391.

4. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto,

Shafiq Alam, An assessment of gold recovery process using crosslinked paper gel, J. Chem.

Eng. Data, 57 (2012) 796-804.

5. Bimala Pangeni, Hari Paudyal, Minoru Abe, Katsutoshi Inoue, Hidetaka

Kawakita, Keisuke Ohto, Birendra Babu Adhikari and Shafiq Alam, Selective recovery of

gold using some cross linked polysaccharides, Green Chemistry, 14 (2012) 1917-1927.

6. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto and Shafiq

Alam, Preparation of natural cation exchanger from persimmon waste and its application to the

removal of Cs+ from water, Chemical Engineering Journal,242(15) (2014), 109-116.

188

7. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, Kedar

nath Ghimire, Shafiq Alam, Preparation of novel alginate based anion exchanger from Ulva

japonica and its application for the removal of trace concentrations of fluoride from water,

Bioresource Technology, 148 (2013) 221-227.

8. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, Shafiq

Alam, Adsorptive removal of fluoride from aqueous medium using a fixed bed column packed

with Zr(IV) loaded dried orange juice residue, Bioresource Technology, 146 (2013) 713-720.

9. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, Kedar

Nath Ghimire, Hiruoki Harada, Shafiq Alam, Adsorptive removal of trace concentration of

fluoride ion from water by using dried orange juice residue, Chemical Engineering Journal, 223

(2013) 844-853.

10. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, Hiroyuki

Harada, Shafiq Alam, Adsorptive removal of fluoride from aqueous solution by using orange

waste loaded with multivalent metal ions, J. Hazard. Mater., 192 (2011) 676-682.

11. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Miyuki Matsueda, Ryosuke Suzuki,

Hidetaka Kawakita, Keisuke Ohto, Biplob Kumar Biswas, Shafiq Alam, Adsorption

behavior of fluoride ions on zirconium(IV)-loaded orange waste gel from aqueous solution,

Sep. Sci. Technol, 47(2012)96-103.

12. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto,

Shafiq Alam Removal of fluoride from aqueous solution by using porous resins containing

hydrated oxide of cerium(IV) and zirconium(IV), J. Chem. Eng. Japan, 45(5), (2012) 331-

336.

13. Hari Paudyal, Bimala Pangeni, Kedar Nath Ghimire, Katsutoshi Inoue, Hidetaka Kawakita,

189

Keisuke Ohto, Shafiq Alam Adsorption behavior of orange waste gel for some rare earth

ions and its application to the removal of fluoride from water, Chemical Engineering

Journa., 195-196 (2012) 289-296.

14. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto and

Shafiq Alam, Development of effective and economical removal technology from effluent

by using orange waste, Proceedings of the Annual Conference of Metallurgists (COM2012)

to be held in Niagara Falls, Sept. 30 – Oct. 3, Ontario, Canada (2012).

15. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto and

Shafiq Alam, Adsorptive removal of fluoride from water by using orange waste gel loaded

with tetravalent zirconium or cerium ions", Proceedings of the IEX 2012 Conference to be

held at the University of Cambridge, September 19 - 21, Cambridge, UK (2012).

16. Hari Paudyal, Bimala Pangeni, K. Inoue, Keisuke Ohto, Hidetaka Kawakita, Kedar Nath

Ghimire, Hiroyuki Harada & Shafiq Alam, Adsorptive Removal of Strontium from Water by

Using Chemically Modified Orange Juice Residue, Separation Science and Technology

(Accepted: DOI:10.1080/01496395.2013.877032)

190

Appendix II

In Preparation and Submitted Manuscripts


Manju Gurung and Shafiq Alam, Development of low cost adsorbents from agricultural

waste biomass for the removal of Sr(II) and Cs(I) from water, (Submitted to Waste

Biomass Volarization)

2. Adsorptive Recovery of Trace Concentration of Au(I) from Model Solution in Sodium

Hypochlorite Media (will be submitted soon)

3. Leaching of Gold from Mongolian Gold Ore by Acidic Thiourea Solution (Manuscript

in Preparation)

191

Appendix III

Participation at Different Scientific Meetings or Conferences

1. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Biplob Kumar Biswas, Hidetaka Kawakita,

Keisuke Ohto, Adsorption of tetrafluoroborate, BF4-, on Zr(IV)-loaded orange waste gel from

aqueous solution, Water and Environment Technology Conference 2010 organized by Japan

Society on Water Environment, Yokohama National University, (2010) June 25-26. (Oral+

Poster)


Removal of tetrafluoroborate ion from synthetic effluents using zirconium(IV)-loaded

saponifiedd orange juice residue, The 5th

Saga University-Daegu University Joint Seminar,

Saga, Japan, (2010) Nov. 17. (Poster) Awarded by gold poster award


Kumar Biswas, Shafiq Alam, Adsorptive separation of tetrafluoroborate from aqueous

solution by zirconium(IV)-loaded saponified orange juice residue, 9th

Japan-Korea

Symposium on Water Environment, Fukuyama, Japan, (2010). (Oral)

4. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita , Keisuke Ohto,

Separation and Recovery of Gold by an Adsorption Processes Using Cross Linked Paper Gel

, The 2nd

Saga University-Liaoning University Joint seminar, (2012) Feb.8 (Poster)


Adsorptive recovery of Au(III) using cotton cellulose treated with concentrated sulphuric

acid, Joint meeting between JSIE and JASE The 27th

symposium on ion exchange and

solvent extraction Miyazaki University, (2011) Nov. 24-26. (Oral)

6. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, An

192

assessment of gold recovery process using cross-linked paper, Joint meeting between JSIE

and JASE The 27 th

symposium on ion exchange and solvent extraction Miyazaki University,

(2011) Nov. 24-26. (Poster)


Recovery and refining of gold by using some cross-linked polysaccharides, Gold 2012, The

6th

international conference on gold science, technology and its application, Keio Plaza

Hotel (Shinjuku Tokyo) Japan (2012) Sep. 5-8. (Oral)

8. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita, Keisuke Ohto, Novel

way of precious metal recovery using some chemically modified polysaccharide gels, Saga

University, Japan, (2012) Nov. 10. (Poster)

9. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Hidetaka Kawakita, Shintaro Morisada,

Keisuke Ohto, Adsorptive Removal of Cesium and Strontium by Using Low Cost

Adsorbents Prepared from Biomass Wastes. 9th

Asia Pacific Conference on Sustainable

Energy and Environmental Technology (APCSEET-2013) Tokyo Inn. Hotel Narita, July 5-8.

(Oral)

10. Bimala Pangeni, Katsutoshi Inoue, Keisuke Ohto, Adsorptive removal of cesium and

strontium by using biomass waste, Hokkaido University, MMIJ Meeting September 2-6,

2013. (Poster)

11. Bimala Pangeni, Hari Paudyal, Katsutoshi Inoue, Keisuke Ohto, Development of low cost

and environmentally benign removal technology for cesium and strontium from water by

using biomass wastes, The 24th

Annual Conference of JSMCWM, Nov 1-4, 2013, Hokkaido

University, (Oral + Poster) (Won the Best Poster Award)

12. Katsutoshi Inoue, Bimala Pangeni, Removal of harmful substances and the recovery of

193

resources using various biomass, Poster presentation and Exhibition at Kitakyushu, Kokura,

October 11-13, 2012.

13. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Miyuki Matsueda, Ryosuke Suzuki,

Biplob Kumar Biswas, Hidetaka Kawakita, Keisuke Ohto, Adsorption behavior of Zr(IV)-

loaded orange waste gel for fluoride from aqueous solution, Water and Environment

Technology Conference 2010 organized by Japan Society on Water Environment, Yokohama

National University, (2010) June 25-26.


Hiroyuki Harada, Adsorption of fluoride from aqueous solution by using metal loaded

orange waste, The 5th

Saga University-Daegu University Joint Seminar, Saga, Japan, (2010)

Nov. 17. (Poster)

15. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita , Keisuke Ohto,

Adsorptive removal of fluoride from water by using metal-loaded orange waste, 9th

Japan/Korea International Symposium on Resources Recycling and Materials Science,

Kansai University, Osaka, Japan, (2011) May 30- June 1.

16. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka kawakita, Keisuke Ohto,

Adsorption of fluoride from water by using acid treated orange (ATO) waste after loadig

zirconium(IV) ions, Joint meeting between JSIE and JASE The 27 th

symposium on ion

exchange and solvent extraction Miyazaki University, (2011) Nov. 24-26.


Enhanced adsorption of fluoride ion using orange waste by increasing the oxidation state of

loaded metal ions, Joint meeting between JSIE and JASE The 27 th

symposium on ion

exchange and solvent extraction Miyazaki University, (2011) Nov. 24-26.

194

18. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita , Keisuke Ohto,

Application of orange waste gel for the effective removal of fluoride from aqueous solution,

The 2nd Saga University-Lioning University Joint seminar, (2012) Feb.8.

19. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Shintaro Morisada

and Keisuke Ohto, Adsorptive removal of fluoride ions from water by metal loaded orange

waste gels, SETAC Asia/Pacific 2012Sep. 24-27, ANA Hotel Kumamoto New Sky, Japan


Alam, Development of effective and economical removal technology from effluent by using

orange waste, Annual Conference of Metallurgists (COM2012) to be held in Niagara Falls,

Sept. 30 – Oct. 3, Ontario, Canada (2012).


Alam, Adsorptive removal of fluoride from water by using orange waste gel loaded with

tetravalent zirconium or cerium ions", IEX 2012 Conference to be held at the University of

Cambridge, September 19 - 21, Cambridge, UK (2012).

22. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Shintaro Morisada

and Keisuke Ohto, Adsorptive removal of fluoride from water by using metal loaded

seaweed (Ulva japonica) adsorbents, The 25th

annual meeting of Kyushu branch, Japan

Society of Material Cycle and Waste Management (JSMCWM), Fukuoka University, (2013)

May 18.

23. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Shintaro morisada,

Keisuke Ohto, Economical adsorptive removal of trace concentration of fluoride from water

using dried orange juice residue marketed as cattle food, 11th

Japan/Korea International

Symposium on Resources Recycling and Materials Science, Kansai University, Osaka, Japan,

195

(2013) June 17-19.

24. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Hidetaka Kawakita, Shintaro morisada,

Keisuke Ohto, Preparation of adsorbent from orange juice residue for the removal of fluoride

from water, 9th

Asia Pacific Conference on Sustainable Energy and Environmental

Technology (APCSEET-2013) Tokyo Inn. Hotel Narita, July 5-8.

25. Hari Paudyal, Bimala Pangeni, Katsutoshi Inoue, Keisuke Ohto, Development of low cost

and environmentally benign removal technology for fluoride from water by using orange

waste, The 24th

annual conference Japan Society of Material Cycle and Waste Management,

Hokkaido, November 1-4, 2013.

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